Let's use these two sets given to explain what is the domain.
Each value from the left set is x, and from the right is f(x).
If we plug any x from the left set in the function, we'll get f(x) that corresponds to it and that's exactly what the arrows are showing.
Domain of the function is, basically, a set of all values x can have.
In this case, it's easy to see, those are all members of the left set (-6, 1, 5, 8), but sometimes this set can have lots and lots of members, even infinity.
