Basically it senses the triangular current waveform in the output inductor and uses that as part of the negative feedback to stabilize the voltage output control loop. It's somewhat simpler to stabilize than a straight voltage feedback control loop.
That particular device data sheet is unusual in that it doesn't show a typical application schematic, so it's not obvious how it's connected and what external components are needed. You might look at some other current-mode regulators from TI or Linear Technology which should have more complete data sheets to explain their operation.
Also here's a more detailed explanation if you are interested.