Air speed, once the turbo, which is a air compressor, supplys a air speed that is higher than that generated by the engines suction on it's own it is said to be boosting, as in amplyfying the air as it is producing more than the engine could command without it purely by the suction created by the pistons moving down.
Small turbo small lag, big turbo big lag, though the advent of roller bearing turbos has addressed this issue of lag to a large extent, so you can now have a big turbo with a lot less lag.
tabetha