What is procedural generation?
A procedural generation takes in some input, usually a seed and a position in some virtual world, and returns a section of the environment there. A procedural algorithm must return identical environments for identical inputs, after all if this was not the case then objects would change position when you loaded that section of the world again.
You can essentially think of it as a giant filing cabinet that you can grab slices of the world from. All you need to do is define what it's contents are.
Another thing that is not strictly required but usually relevant is continuity between these slices. Sections of the world that are close together should be similar to eachother. They should have similar elevation or you will have abrupt jumps in height, they should have similar temperment or you may have a desert next to a taiga, etc.