Action is rather what it sounds like. It is when the characters are doing something vs. talking about it. For many people it's the good stuff. Fights, sex scenes, car chases, these are all examples of action. So lets go into the basic principles.
Starting with fights. There is one really obvious part of a fight that actually drives my whole explanation for how to approach action.
Fights hurt.
I know right, well duh! Except it gets skipped over all too often. James T Glen points this out in " them's fightin' words"; describe the damage involved, hurting a knuckle on someone’s face, getting a cut, a broken lip. Pain is an important part of what makes it believable.
Translate that to all action and you have:
The bodies reaction.
A car chase is exciting and scary, nerves get rattled, passengers gasp in fear. Watch the Bourne Identity and pay particular attention to the sounds of Marie, the squealing tires, the cars honking, the crunching crashes. Details like this are what you need to make action work.
If multiple characters are involved you need to include them, even if you're writing from a single perspective.
Pay attention to pace. Most action should create a sense of urgency in the reader and use dramatic words.
The best way I have to explain is by example.
Jason could barely hear Marie's gasp of surprise over the sound of blood in his ears, his heartbeat pounding through his body as he grabbed the emergency break and flipped the car. Horns screamed at him relentlessly as sweat beaded on his forehead. He couldn't make it over to the other side of the road. He would just need to barrel through no matter the consequences.
He squeezed his fingers against the steering wheel, his knuckles losing all color as he swerved and swayed - barely dodging a Volvo before the driver jerked it out of the way and spun out. The sound of shattering glass and crunching metal echoed through Jason's skull as the rear-view mirror revealed the pile up he had just left behind.
He had no time to think of the lives that had been lost. It was escape now, or die trying.
And that's an action scene.