The bishop retreats, since taking on f6 would accomplish nothing. Now Black has the idea of playing ...g5, but he shouldn't be too eager to weaken his kingside right away. The threat of breaking the pin is enough to keep White from playing e4 for now, so Black should address the center. 5...c5, forcing White to advance his d pawn, is the most common move, but 5...b6 is also playable, with Black either fianchettoing after 6. e3 or pushing c5 after all following 6. f3.

Wikipedia has related information at Nimzo-Indian Defence.