Generally, clips are devices into which cartridges are placed, for the purpose of quickly charging a magazine, via an adapter, often referred to as a spoon. So, one would carry magazines to quickly reload their weapon during a battle. After the battle, one would pull the clips (stripper clips, in this case) from their pack or what-have-you, and would quickly reload their magazines using the clips and the spoon.
In the case of the SKS (and most WWI & WWII era bolt rifles) the clip holds cartridges to be loaded into the rifle's internal magazine. It's simply quicker and handier than loading one round at a time.
In the case of the M1 Garand, an "en bloc" clip is used, which is a metal clip which is entirely inserted into the rifle, which holds the cartridges in place, and allows the rifles internal follower to advance the cartridges into the chamber.
Hope this was helpful. Someone else will likely come along and explain it better than I have.
Tim