@Bojidar: Nobody was faster so far apparently ;)
@genete: Please try to be clearer in your question next time, nobody knows every single piece of text that is written in some node of the editor, so you should give the context yourself, not wait for others to ask themselves for 5 minutes what you are referring to with "Custom Constants".
My natural answer to the question as you wrote it would be:
They are constants which are user-defined (custom).
You can define constants in GDScript with:
const MYCONSTANT = 42
Obviously that's not what you want to know, but that's the kind of half-trollish answer your question calls for ;)