RPGMemes

Other guy gave an okey explanation, but to try my hand at explaining:

On a typical round of combat, you get three actions. You can spend them in a variety of ways. An attack is one action, movement ("stride" action) is one action, most offensive spells are 2 actions, etc.

A lot of classes get ways to "discount" actions. For example an early feat fighters and barbarians can take is "Sudden Charge" which let's them stride twice and attack an adjacent creature and costs 2 actions.

The whole thing lends so much freedom and takes a lot of burden off the DM for needing to homebrew / make up things on the fly. The whole system is very crunchy though (very detailed and particular on its rules) and so doesn't fit everyone's vibes.