Topping is removing the main stem's growth of a plant at a certain point along the stem to release hormones throughout. It "confuses" the plant about where its highest plant is, and so it sendsa shitload of hormones everywhere instead of just throught its main cola and this makes it grow far more vigorously all around, but does impede verticle growth of the main stem.
Most people agree that whether or not you should top is dependant on the strain of plant you're using, some respond badly while plants like blueberry supposedly love it.