This is a skill that allows the user (P2P or F2P) to help with their combat. There are a number of ones that will make your combat and quests go easier. There is a good section in the WIKI that explains this skill.
Leveling up the skill outside of the rewards of the various quests takes a lot of time and effort, or, a lot of GP. While playing, Zelda and myself found a way for P2P players to bring up the skill a bit faster. If you hunt werewolves in Canifis (you have to finish the priest in peril quest) save the wolf bones that they drop. At the grand exchange sell all of the bones and then buy dragon bones. Wolf bones gives you 4.5 XP per bone, dragon gives 72. At this time you can sell 5 wolf bones and get one dragon bone. Using a bit of math you can either get 22.5 XP burying 5 wolf bones or 72 for one dragon bone. If you have a house (POH) with a high level altar and incense burners or access to one then you can get more bonus points. Read the section in the WIKI on the POH.