Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...h6

3... h6
Black plays h6 to prepare for the move 4...Nf6. If Black played 3...Nf6 instead, White would respond with 4. Ng5 with the intention to take on f7, winning material and turning the game into a Fried Liver Attack. With 3...h6, 4. Ng5 would no longer be possible and Black can play 4... Nf6.

However, this move might be considered to lose tempo as White can utilize this slow play by playing 4. d4, striking back at the center. If Black insists with playing 4... Nf6? anyway, Black would be losing the e5 pawn.

Theory table
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'''1. e4 e5 2. Nf3 Nc6 3. Bc4 h6'''