It means bitshift left, so:
int i = 1 << 2;
// 0000 0001 (1)
// shifted left twice
// 0000 0100 (4)
A left bitshift is analogous to multiplying by two, and a right bitshift acts as a divide by two.
Bitshifts are useful because they convey semantics better when working with bitmasks and they are (on x86 at least) faster than multiplication
