Meta, short for metagame, is a term for the knowledge of how a player or moderator will act based on previous experience. It only makes sense that people will pick up on, say, an ordinarily active player lurking in the games where they are scum. Likewise, some moderators state outright that they will not use Cops.
The application of this kind of knowledge is called metagaming, or more strainedly "metaing". This is simply extrapolating the existing meta knowledge to the current game. In the case of the active player who only lurks as scum, it makes sense to extend that to say that if they are lurking in the present game, they are more likely to be scum and should be treated accordingly. Likewise in the case of the moderator who does not use Cops, if one of the players in their game claims Cop, players will be much more apt to consider it a false claim.
Players who manage to bring about the appearance of looking and behaving the same way regardless of alignment become tough to read, as they are best judged by their actions inthread. This takes time to do, which means that these players are frequently not lynched Day 1.
Of note is that metagaming works only as far as the player is unaware of their own meta, or is unwilling to break their own meta. In the instance of the player who lurks as scum, if that player lurked in a game and flipped Town, this would indicate that the meta tell is no longer valid, as the player is actively trying to "mix up" their play (or they could have just had no time to play during that game - it's worth asking about).
It is still possible to use old meta tells, though, as players have a tendency to revert to their original play style under pressure.
Another potential flaw for metagaming is that after a single game, players will attempt to pick up on something notable their meta target did or did not do and associate it with their target's alignment when the target would actually do that regardless of whether they were Town or scum. This tends to lead to frustration after a player wins as scum by doing nothing out of the ordinary, or when a player with an established Town playstyle tries to play differently in a later game as Town.
After getting burned by the limitations of meta - or perhaps because they are particularly susceptible to meta tells - some players ardently feel that metagaming doesn't work. There is no denying that, at least in some situations and to some degree, it does though. To that end, once players establish how to play and others start to recognize them, they will have to deal with obscuring whatever meta tells they have. For players who prefer to be Town, this leaves them with the unpleasant choice of improving their scum game (easier said than done) or watering down their Town game. On the other hand, players who prefer to be scum usually just need to improve their Town game.
"Self meta" or "personal meta" is the practice of referring to your own tendencies in game, usually for the purpose of trying to 'prove' yourself Town or disprove an argument that you are scum. Because the player is obviously aware of their own tells, however, and their true alignment is not yet known in that game, self meta is best viewed with at least a shaker of salt; they could be playing to avoid their own tells, after all. It also runs the risk of crossing the boundary of talking about ongoing games.