Bug Abuse is the act of finding a glitch or bug in the game by accident or intentionally and exploiting it for an unfair advantage in the game.
The most notably instances of Bug Abuse were the World 111 Glitch which allowed players to attack anyone outside of the wilderness and the Party Hat Duplication Glitch which allowed players to duplicate items, specifically party hats. Some bugs which may not be classified by Jagex as Bug Abuse include the infinite prayer glitch which Jagex is aware of and has determined that it actually provides no unfair benfits to the player using it other than a prayer symbol abover their head because a player cannot move or do anything while using it. So it is not possible to use it in combat. Other bugs which aren't exploitable and therefore aren't against the rules to use include the POH chair glitch and the See Through Cooking glitch because they provide no unfair advantages in the game. The Cooking See-Through glitch actually is considered by Jagex not to be a glitch as it is normal to be able to see through a player while cooking over a range.
It should be noted that finding a glitch in runescape is not against the rules unless you choose to exploit it without reporting it to Jagex. If Jagex discovers you chose to exploit a glitch without notifying them then you can be banned for Bug Abuse.
If you do stumble across a glitch in the game the best thing to do is to report it to Jagex by using the "Report a Bug" link on the main page as soon as possible.