In high school, I bought Unreal just to play with the level editor.
Just after college, I bought Crysis to learn Lua.
Mods are “modifications” of an existing game. I modified the rules in Crysis to make your guy jump really high, or punch 10x harder.
I made simple mods by exploring and editing C++, XML, and Lua files.
Games that support modding or custom levels will often provide tools for you! Mods are a great place to start if you want to learn to make games, or even pad out your game design portfolio.