The French defence is a chess opening, one of the asymmetric replies to 1.e4, namely 1...e6. It is the third most popular reply to 1.e4. Often, Black plays more actively on the queenside while White plays more actively on the kingside.
A problem that Black often has in the French defence is finding a way for his light squared bishop to become active. Although this is true, it may not matter if Black knows what he is doing. The defence has a reputation for reliability, and has been played by many great players (such as Botvinnik), and has featured in several world championship matches.
Typical examples go:
Tarrasch variation change
1.e4 e6 2.d4 d5 3.Nd2 c5 (3...Nf6 is popular) 4.exd5 exd5 5.Nf3 Nc6 6.Bb5 Bd6 7.dxc5 Bxc5 8.0-0 Ne7 and so on. This line is quiet, and mostly about the centre.
Winawer variation change
1.e4 e6 2.d4 d5 3.Nc3 Bb4 4.e5 c5 5.a3 Bxc3+ 6.bxc3 Ne7 7.Qg4 Qc7 (7...0-0 is possible) 8.Qxg7 Rg8 9.Qxh7 cxd4 10.Ne2 and so on. The play in lines like this is strongly asymmetric: White owns the kingside, and Black owns the queenside.