topping is used for cases where you have limited vertical room and need the plant to bush out more than grow up.
the easiest way i can show you is to imagine the plant like this \l/ where you have 2 branches in the main stem.
you top and are left with \ / and the main grow sites are the 2 top most branches.
I wouldnt worry about it too much, if you know your strain you have an idea of height tendencies. If you do top it streches the veg stage because the plant has to go thru a large adjustment.
so for now , i dont think it should be a concern for you...what strain do you have?