If you have ever played Exalted then you know what stunting is. If not, keep reading. If you describe your action as “I lift my sword above my head, bring my shield up in a defensive position, and swing down using the full force of my arm and body across their torso,” I may choose to give you a +1 bonus on your attack and/or damage roll. If you give me a really detailed description that involves really good cinematic language and I think you have done an exemplary job, I may increase the bonus to +2 or +3. This will reduce some of the pressure of describing actions from me, and thus I will reward your efforts.
Important note: you must be able to perform the description. If you want to hit them with your shield then you shield bash, if you want to jump across the table grab the chandelier and swing behind your enemy… get ready to make a lot of different checks.
This can be used out of combat too. If you are picking a lock for example you might describe how you “produce a small pin from beneath your tongue, analyze the lock for a moment, and then bend the pick into the proper shape.”