Repository
A repository is a set of code that is kept in one specific box so that programmers can manage it easier.
Why?
Repositories are deeply tied to version control. The problem with code is that its really difficult to manage - and version control solves that.
A metaphor
Imagine that you are moving to a new house. When you pack up your belongings, you may do something like "this is the 'kitchen' box" and "this is the 'bedroom' box" and so on.
Those boxes are repositories. You can pick them up and take them somewhere else and know what is in it when you get there. Additionally, the stuff inside the box won't change when you move it around and put it somewhere else. Unless you open the box up and change some of its contents, you can sleep soundly knowing what that box has in it.