Question about CHAP (Challenge-Handshake Authentication Protocol)

So as I know, In CHAP the client will send the request to server and the server will send the challenge back to client and then client will
do the challenge and send the result back to server, if the result from client is the same as server, the client will have access to information.
but what is challenge ? Is it hash function or just random text or something else I don't understand.