What is Topping?
Topping is a way of high stress training (HST) your cannabis plant to fill out more like a rounded bush and less like a Christmas tree with one big central main bud and smaller side and bottoms. Topping is especially important in scrogging but useful for every grow to improve yield overall.
Topping is also pretty easy to do as well! During veg, every few nodes you just clip away new growth sites. The growth will focus outwards instead of upwards, and before where there was only one whorl of new growth, two will grow in its place. I can appreciate that it may seem weird to clip a new growth site, but the two new sites will pop up in a couple days, fully out in a week. You can top as much or as little as you want. I start when the plant is 3-5 nodes high (it's personal preference) and continue every few nodes after that for the whole veg cycle.
Topping creates an even canopy, with growth and light energy distributed evenly through all the developing stalks and colas. Without topping, the main cola will receive the most light, and shade out lower branches. Usually the buds on the sides become much smaller than the central cola and the buds at the very bottom aren't even worth the energy to trim (even if they are covered in trichomes).
What is FIMing?
FIMing (which stands for "fuck, I missed") started out as a mistake but actually turned into a valuable pruning method. It's exactly the same process as topping, except you are cutting mid-whorl instead of at the base of new growth as you would in topping. Where topping will produce 2 new growth sites in the place of one, FIMing would produce 6-8 new growth sites. While topped plants will produce a large and bushy structure, the FIMed plant will require more structural support and trellising (again great for SCROG).