Basically photons have a quantum mechanical property called "spin." If you measure spin, it can take on one of two values (up or down, for example), just like a classical bit can be 0 or 1. It's a powerful tool powerful because in quantum mechanics, a qbit can be be both up and down at the same time during a computation whereas a classical bit has to be in either the 0 or the 1 state. This allows much faster computation of some complicated problems.