In the opening, when the black knight goes to c6 white often responds with bishop to b5. What is the purpose of this move?
For example the Ruy Lopez/Spanish opening:
[FEN ""] 1. e4 e5 2. Nf3 Nc6 3. Bb5
This opening is called the Ruy Lopez or the Spanish Game.
It is actually quite logical: 2.Nf3 attacks the e5 pawn and Nc6 defends against this attack. Now Bb5 attacks the defender, which renews the attack on e5. For tactical reasons the threat of taking on c6 and then taking on e5 isn't acute yet, but the idea is nonetheless to exert pressure on the black pawn on e5.
Additionally 3.Bb5 makes it possible for white to castle in one of the next moves. This is another motivation for an early kingside development.