Like Lent, Advent is a penitential season, so the priest normally wears purple vestments. But on Gaudete Sunday, having passed the midpoint of Advent, the Church lightens the mood a little, and the priest may wear rose vestments. The change in color provides us with encouragement to continue our spiritual preparation—especially prayer and fasting—for Christmas.
For this same reason, the third candle of the Advent wreath, first lit on Gaudete Sunday, is traditionally rose-colored.
Gaudete Sunday has a counterpart in Lent: Laetare Sunday.