GetButtonDown Button-Names ..?!

I’ve seen stuff like


And it works in my file where I am following a tutorial.
But where is the jump-button defined? How does it know it’s [space]??
Where can I find a list of these kind of definitions if there is one?

You can see the definitions, and define your own, by using the Input Manager.

You can get to it from the main menu using Edit->Project Settings->Input.