The cassock traditionally has 33 buttons up the front, each representing one year in our Lord's life. Each sleeve has 5 buttons, one for each wound Jesus received during His crucifixion. The "Roman collar" is a sign of the priest's willing obedience to Christ and His Church.
The cassock is a sign to all who see him that he is a priest and servant of the Lord Most High.
(Posted by Dana Acly to Keeping Catholics Catholic on Facebook).