What are exactly Laravel Contracts?

I'm new to Laravel & I wanted to know something about it's feature that calls Contracts.
(If my question not in place, let me know why, and don't just downvote it).
So from what I red in Laravel Documentation and say on Laracasts videos, I understood that contracts they are only interfaces for class implementation.
So what it's good for? That if I or someone else will implement those interfaces will all need to go by the interface and then I dont need to change my code at all?
Is that the reason why Laravel uses it's implementation as a contracts ? Also I wanted to know, to achive the implementation I must bind the implementation to a contract?

via Chebli Mohamed