Topping plants is when you chop off the top stems head. Check out the photo below.
This is very easy. Just cut the head off and two new stems will come out of the sides where you cut the weed plant. These two new stems were going to grow out later, but when you cut the head it forces them to grow instantly.
Topping marijuana plants is very popular, however fimming is a much better choice in my opinion. Topping forces two new stems, fimming saves the top and forces the two new stems anyways. Play around with it and see what you prefer. Some people will never stop topping thier plants, I prefer fimming but thats my choice. Check out the fimming guide and see what you prefer. Fim vs Top = i like fim. I was reading on the forums that some guy compared the two and said plants that were not topped have a bigger main cola. The main cola is the largest and tallest bud on your plant. I haven't tried for myself, but this guy says NOT topping gives you that bigger bud. In my opinion this makes a lot of sense. When you top the plant your REMOVING the main colas stem completely. This means you murdered your main cola, LOL. Fimming or topping technically does slow your plants growth by 1-2 days. If you want to avoid topping or fimming and still get loads of branches and colas and keep the main cola, BUY SUPER FOLIAGE SPRAY PLANT STEROID. This spray will sky rocket your plants growth and give you tons of branches and colas with no cutting involved!