Unrelenting Force is the first Dragon Shout or Thu'um that the Dragonborn will learn in his adventures.
The shout is made up of three words Fus, Ro, Da meaning "Force, Balance, Push" in dragontongue. This shout is useful for knocking back powerful enemies, blasting enemies and items of cliffs, as well as many other uses a player could find.
The first word of Unrelenting Force Fus, merely makes a small enemy stumble.
The second word Ro, isn't much of an improvement over the second but can make some larger creatures stumble slightly. A common tactic with this is to use it against a dragon as its about to use a breath attack as it stops it and has a low cost.
The third word Dah, is very strong and sends small to medium enemies flying and staggers large creatures.