Definition of "know what is what" in English

Verb

To understand the current situation; to know what is going on.
- He seems to have no trouble understanding stuff that, to me, sounds like hot garbage. He really does know what's what.
- She said she was going to house to study French, but I wasn't born yesterday. I know what's what.
- Everything is changing so fast... I just don't know what's what anymore!