On your turn, heroes get 3 actions to attack, move around the battlefield, cast spells, etc. Generally doing any single thing in combat costs 1 action. Some very strong spells or special abilities may take more time and thus cost more actions.
All 3 actions recharge at the end of your turn, so there’s no need to save them up. Spend them all on your turn and when you’re done, you get them all back!
Free Actions These do not cost an action or any other resource (e.g., mana) unless otherwise specified. Heroes can do simple tasks (open an unlocked door, shout a simple phrase, drop an item, end concentration, etc.) for free 1/ turn.