Simply worded, a sump is just a secondary tank that is set-up and linked into a main / display tank as an option for placing equipment as well as providing filtration for the main tank.
If I have learned anything in this hobby it is that there are many many different ways to achieve a healthy and thriving tank. This could not be any truer with how you choose to set up a sump. Likely the best way to approach this topic is to describe the factors that I learned which go into designing your sump and then show you how I set up my sump just as one example. I hope you will find these sump basics helpful.