A closure is like that one relative of your crazy aunt, always lurking in the shadows, waiting to trip you up.
It's a function that has access to its own scope, but also to the scope of its outer functions. Like, imagine you're at a high school reunion, and you bump into your old friend from the math class. But this friend is also secretly married to the teacher who taught you calculus... it's like that.
There are two kinds of closures: lexical and block.
So, which one do you want to learn about first?