RAI, the acronym of "Rules As Intended", is any unofficial attempt by the gaming community when to figure out what the game developers "meant" when they wrote a rule. This is in contrast to Rules as Written.
RAI usually comes up when a rule in a game is either impossible to obey, inexplicably obtuse, or seems completely at odds with other aspects of the game. RAI can also come up in discussions about multiple game mechanics interacting poorly with each other in a complex game. RAI, by its nature, will generate a more diverse variety of interpretations of a single game mechanic than RAW; inevitably leading to more skub.
Common RAI discussions crop up during rules changes in Warhammer 40,000 or Warhammer Fantasy. While the core rulebook for any given edition is likely to be well-edited and nearly singular in interpretation of the rules therein, other supplements are not. Codecies (particularly those for less popular armies and those which are holdovers from previous editions) can introduce faction-specific rules which are over-complicated, unspecific, or fail to mesh with other aspects of the game. In rare cases, some rules meet all of those criteria at once! Other supplements (such as campaign books) will explore niche factions or make modification to a "parent faction's" ruleset, both of which are common sources of obtuse rules. /tg/ will rage over such things for weeks, if not years.