It means that that the speed/power being utilized by the tool changes with the degree of force you put on the trigger. Depress it 50% of the way and get 50% of the power in the selected mode. Pull it all the way back to get full power.
It’s useful for easily (but not very accurately) adjusting torque and RPM on the fly. If being free to change the power mid-motion is amongst the more important categories for your criteria, search for products that specifically tout this function when placing your order. This way you can make sure you like the result.