Why does pastor wear that?

Alb
The alb (from the Latin albus, meaning “white”) is an ample white garment coming down to the ankles and is usually girdled with a cincture (a type of rope). It resembles the long, white linen tunic used by ancient Romans. It is worn over the cassock, but underneath any other special vestments, such as the stole, dalmatic or chasuble.

The alb is a representation of our white baptismal gown, the covering that shows our sins have been washed away in Christ.

Prayer when put on: “Dealba me, Domine, et munda cor meum; ut, in sanguine Agni dealbatus, gaudiis perfruare sempiternis.” (Cleanse me, O Lord, and purify my heart, that, washed in the Blood of the Lamb, I may attain unto everlasting joys.)
Stole
The stole symbolizes priestly authority. It consists of a band of colored cloth, usually of silk, about seven and a half to nine feet long and three to four inches wide. The stole is almost always decorated in some way, often with contrasting galloons (ornamental trim) and fringe is usually applied to the ends of the stole following Numbers 15:38–39.

Putting on the alb, your pastor recites this prayer: “Redde mihi, Domine, stolam immortalitatis, quam perdidi in praevaricatione primi parentis; et, quamvis indignus accedo ad tuum sacrum mysterium, merear tamen gaudium sempiternum” (Lord, restore the stole of immortality, which I lost through the collusion of our first parents, and, unworthy as I am to approach Thy sacred mysteries, may I yet gain eternal joy).
Pectoral cross
The pectoral cross or pectorale (from the Latin pectoralis, “of the chest”) is a cross that is worn on the chest, usually suspended from the neck by a cord or chain. Some contain a corpus like a crucifix while others use stylized designs and religious symbols.

In our denomination, it is only worn by ordained clergy (although anyone is allowed – and welcome – to wear a Christian cross on a chain). It is a reminder that the pastor is to deny himself (leave the worldly desires behind), take his cross (sorrows, burdens and worries) and follow Christ in all that he does.

The prayer I say when I don this: “Lord, give me strength to heed your words: ‘If anyone desires to come after Me, let him deny himself, and take up his cross, and follow Me.’ Please conform my will to yours and bless my words and actions.”
Cassock (aka Soultane)

A long garment worn by clergy both as ordinary dress and under liturgical garments. The cassock, with button or snap closure, has long sleeves and fits the body closely. The clothing has 33 buttons or snaps, to signify the 33 years of Jesus’ life.
  The cassock derives historically from the tunic of classical antiquity that in ancient Rome was worn underneath the toga, and the chiton that was worn beneath the himation in ancient Greece. In religious services, it has traditionally been worn underneath vestments, such as the alb or surplice. Pastor wears it to signify that he, like his congregation, is a sinner.

When your pastor dons it, he says a short prayer: “Dóminus, pars hereditátis meæ et cálicis mei, tu es qui restítues hereditátem meam. (Lord, the portion of my inheritance and my chalice, You are He who will restore my inheritance)”
Surplice (superpelliceum)
A surplice is a tunic of white linen or cotton fabric, reaching to the knees, with wide or moderately wide sleeves. It was originally a long garment with open sleeves reaching nearly to the ground. It is appropriately worn by any pastor, by lectors and acolytes, or indeed by altar servers who are technically standing in for instituted acolytes for any liturgical service. It is to be worn over a cassock and never alone, nor is it ever gathered by a belt or cincture.

The surplice is meant to be a miniature alb, the alb itself being the symbol of the white garment received at Baptism.

Prayer when put on: “Indúe me, Dómine, novum hóminem, qui secúndum Deum creátus est in justítia et sanctitáte veritátis” (Invest me, O Lord, as a new man, who was created by God in justice and the holiness of truth.)
Chasuble
The outermost garment worn by some pastors when they celebrate the Eucharist. The chasuble is a conical or bell-shaped cloak made from a semicircular piece of cloth sewn partially up the front with an opening for the head; the related item of clothing called a cope is not sewn in front but is closed with a clasp. It developed from an outer garment ancient Greeks and Romans wore, called the paenula or casula (“little house”). It was worn by both laity and clergy until the 6th century, eventually becoming specifically an ecclesiastical vestment. Pastors only wear a Chasuble for the Eucharist. It is put on just before the service of the holy supper begins, as a note that we have left the service of the word (bible readings and preaching) and have entered the third and final service of the day (the service of the Lord’s Supper).
Maniple
The maniple is a liturgical vestment, an embroidered band of silk or like fabric that hangs from the left forearm, worn over the alb. It is used only during Mass. The original purpose of the maniple is unclear, but it probably originated as a cloth that the priest could use to wipe his hands and face. The maniple has two purposes: to be a representation of the rope by which Christ was led, symbolic of the tears of penance, the burden of sin, and the fatigue of the priestly office. It’s second purpose is to represent the towel that Jesus draped over his arm to wash his disciples’ feet. Pastor currently only has an advent maniple, but he would like more!

Prayer when pastor puts it on: “Merear, Domine, portare manipulum fletus et doloris; ut cum exsultatione recipiam
mercedem laboris. (May I deserve, O Lord, to bear the maniple of
weeping and sorrow in order that I may joyfully reap the reward of my labors.)”