The Definition of a Set
Easy, right?
Just think about the name.
A set. A collection of things.
Indeed, a set is nothing more than a collection of some stuff, objects, numbers, things, whatever.
Everything that will be collected, part of a set, will be an Element of that set.
To say that something is part of a set, you will use the "in" symbol:
With that, it makes perfect sense to learn about the next concept: