See: WHAT'S WHAT.
{n. phr.}, {informal} 1. What each thing is in a group; one thing from another. * /The weeds and the flowers are coming up together, and we can't tell what is what./ 2. All that needs to be known about something; the important facts or skills. * /Richard did the wrong thing, because he is new here and doesn't yet know what's what./ * /When Bob started his new job, it took him several weeks to learn what was what./ * /When it comes to cooking, Jenny knows what's what./ * /Harold began to tell the teacher how to teach the class, and the teacher told him what was what./ Compare: WHICH IS WHICH, WHO'S WHO.