Here is my best explanation of what they actually are and why they matter:
It takes effort to get AI to do exactly what you want. You want it to write a scope of works?
First attempt - bad
Give it feedback, missing context, incorrect instructions
Second attempt - better
More feedback, more context, include an example
Third attempt - pretty good
That cycle is annoying to go through everytime you prompt AI. So instead you do this once, capture what you have built as a re-usable skill
Then everytime you get AI to write a scope of works, it will give you more consistent/better results