A set is in between an exercise and a package.
In general, the difference is as follows:
Sets: Sets contain exercises that build upon one another, like sub-steps in a question. For example, when you have question 1a, 1b, and 1c.
Packages: A package is a collection of exercises, which might not have anything in common. For example, when you have question 1, 2, 3, and 4. A package can contain both single exercises and sets of exercises.
Sets are especially useful when you randomize variables. If exercises are related, and you need the outcome of one exercise to build upon that in another exercise, it does not make sense to generate new variables at every sub-step of a question. In that case, you would put the exercises in one set. Sets make sure that the exercises in the same set share the same random variables.
Sets do have a unique ID number, but no name.
Lastly, it's good to keep in mind that every exercise is actually in a set. But most exercises will be the only exercise in their set.
Watch a video to see it in action: