Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6/5. d3
Though passive and harmless-looking, this move defends the e4-pawn and makes Bxc6 followed by Nxe5 a reality. Therefore, Black has to loosen his queenside with 5...b5. White doesn't want to attack immediately. He wants to attack after building an impregnable fortress that offers no chances for counterplay.
- MacDonald, Neil. The Ruy Lopez, Move by Move. Gloucester, 2011