3. Bb5, black plays
a6 puts the question to the bishop. If he takes your knight, recapture with the
d pawn. Black has a very strong game at this point. Normally you capture toward the center, but in this case, you can play
c5 and control the
d4 is very important to control in the Ruy Lopez.
What you see most often after
a6 is the bishop retreating to
a4. White may play
c3 later in the game to give his bishop a square (
c2) to back up into. You see the same thing happening on the other side of the board with
f3 in the Queen's Indian, giving white's bishop on g5 the option to back up to
Bh4 and then retreat to
c3 also prepares for the
d4 push, common for white in the Ruy Lopez.
c3 protects the
You're probably going to see
Nf6 eventually. After
Bb5 you might see it right away. Or you'll see it after
3. Bb5 a6 4. Ba4 Nf6. What does
Nf6 do for black? It attacks the unguarded
e4 pawn with the knight. But white doesn't have to worry here. Instead, white castles
Take a moment to play this out on a board:
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Nxe4 6. Re1
Black is screwed after
Re1. Notice that the rook is staring down the king. The pawn/knight are pinned. If black retreats the knight back to
Nf6, now white takes the knight. Black recaptures the bishop. Now the pawn on
e5 is unguarded.
Nxe5 and you have a deadly discovered check on black's king when the knight moves.
1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Nxe4 6. Re1 Nf6 7. Bxc6 dxc6 8. Nxe5
it's all over for black.
That's how Capablanca describes the Ruy Lopez in his book Chess Fundamentals (if I remember correctly).