Pruning help

ok well im not exactly looking to prune my plant, but ive been reading trying to get an idea of wat you do and i just dont get it...wat exactly are u cutting?(could u be a little more elaborate then calling it the grow tip), when would u usually do this?

basically im trying to understand how this works just in case i find myself in a position where I have to do so.

By pruning do you mean Topping? FIM'ing? Cloning?
The only real reason to FIM or Top is if you're short on space and want them to bush out a little more than just growin up like they normally do.

basically where a new set of leaves comes out is called a node,then out of that comes new growth(another set of leaves) thats what you cut off the little stem if u cut it all the way down somewhat even with the node thats topping (i think) if you leave a little of the stem above the node like a 1/8 of an inch maybe a little more thats fim
and like zombi said its to make them bush out