Off the top of my head,
Essentials: pot odds/implied odds, chances to hit an out, odds of common all in scenarios (coin flips, domination, overpair vs underpair, etc.)
Intermediate: Ranges, Expected value, Equity, Equity vs opponents ranges, Combos, Card Removal
IMO the most important is getting a feel for your equity in common scenarios, for example – Flopped a flush draw vs tight opponent on Axx board, Flopped a TPTK vs LAG in early position, etc. Use Equilab for free if you want to get started
