OK.
Let me see if I can describe this in an easily understandable way.
Figured since this is a hacking website, most if not all, already had a sort of fundamental understanding towards what CE and CT is and how to use it.
That's basically where most of us here began.
So basically, a .CT (Cheat Table) is a saved file containing references to memory locations in your RAM.
Which is created by an application called Cheat Engine (or CE for short).
When a game is active in your PCs memory it has to somehow manage the information going on, right?
Example, if you walk somewhere in a game, there's something handling that variable or in that case something that translates to in-game coordinates.
If you were to walk in any direction, it'd change that variable (Number).
Same goes for your HP, MP or whatever else is contained within a game.
The game in some way handles this information.
That's usually what's in a .CT file, and they are opened using Cheat Engine.
You can also make your own inside Cheat Engine.
The CT's you download from here are other peoples work, they have found these adresses and possibly coded scripts in order for this program to in some way alter how the game is running. Such as giving you infinite ammo, no reload or whatever else was included in what you'll download or make.
If you're downloading it's usually as easy as opening the CT with CE and pressing a checkbox to get either, or just change the values you want accordingly.
If you're making your own, it'll take some time, there's a tutorial included with Cheat Engine.
I recommend you try it out.