A typical move, preventing …Bg4. It's not really necessary and it's probably not the most accurate, despite being the most played.
1. e2e4  1... c7c5  2. g1f3  2... b8c6  3. f1b5  3... g8f6 Caruana used this move faithfully during the Candidates Tournament. 4. b5c6  4... d7c6  5. d2d3  5... d8c7 