Since you're new to PokerTracker I assume you're using PT4 (although you posted in the PT3 section). You can see what each stat is in the tooltip if you hover the mouse over it in the Hud, or by looking in the Hud Editor (Hud > Edit Hud Profiles).
You can see exactly how each stat is defined by going to Configure > Statistics. Select the appropriate game type (cash or tournament) and look at the "Player" section for stats which are used in the Hud. You can type in the Search field to find specific stats more quickly.
If there's anything specific that you don't understand please ask.
Quick Start Guide