the roll results are the result of a hash digest acquired from a hash function. what you feed to that hash function determines the result. so the point of these systems is to come up with a message (to be hashed) which is a concatenation of two or more parts that neither party could predict the result to.

so what the server does is that it generates a random seed, then doesn't reveal that to you. it only reveals the hash of it. this ensures that the server can not decide to change it later but at the same time you will not know what the seed is so you also can't predict the result of the final hash (like keep selecting different seeds to come up with your desired result and then publish your own seed).

then you come up with a seed and share it with the server, it combines your seed with their seed and runs the hash algorithm on it to come up with the final result. then they publish both final result and the seed so you could now verify it.

by the way, it might be a typo but the term is "provably" meaning "capable of being proved" not probably.