Angleshooting is defined as doing things, or applying rules, in ways they weren't intended in order to gain an advantage in a game. In mafia, a common example of this is using OGI (out of game information) to exert public pressure or influence on the game, to publicly give reads based on OGI, or in more extreme cases, to use information acquired through illegitimate means to covertly influence a game. Angleshooting also covers all intentional uses of any flaws/quirks in the forum software in order to subvert the spirit of the game.
This definition is purposefully non-specific because angleshooting is more of a general idea than a specific case. Generally speaking, you know it when you see it, and it can range from relatively innocent/harmless to malicious and blatant cheating.
A good rule of thumb is if you're attempting to push a game in a particular direction, and your reasons for doing so are not based on what's directly happened in the game, or meta, you're in the danger zone.
The game of Mafia is firmly against angleshooting. If you are unsure if a particular action/read is angleshooting or not, (privately) ask the game host. If you encounter something in a game that you believe to be angleshooting, inform the game host and the game host will take a look at it.