I never read the books and only played the games. I do not understand forerunners. Why does 343 guilty spark say to the master chief that he is forerunner? I thought people said that forerunners and humans are two completely different things?

they are but honestly you are going to have to read the books.

The monitors believe that humans are of Forerunner descent.

Guilty Spark has always been confused, you shouldn’t trust much that he says.

The Forerunner left behind their technology for the humans to inherit, to follow the protocol of something known only as “The mantle”. The mantle is the idea that a species shall succeed another and inherit its technology and knowledge. This is why only humans can active the Halos.