What’s what
Definition 1 of 1
Pro Tip 1/3Grasp the Basics
Use it when showing that someone understands the basic principles or the core knowledge of a subject. 
After a few guitar lessons, she's got what's what down.
Pro Tip 2/3Insight into Reality
Expresses that someone has a clear understanding of a situation's reality, beyond its surface appearance. 
In the business meeting, she quickly showed she knew what's what.
Pro Tip 3/3Not for Instruction
It's not usually used for giving instructions, but rather for acknowledging someone's understanding or expertise. 
He doesn't need guidance, he knows what's what.