The Hated modifier makes it so that a player requires one less vote than normal to be voted out, or eliminated. For example, if there are seven players alive, most players will require four votes to be eliminated but a Hated player will only require three.
Some bastard mods will use this as a hidden modifier.
The Hated modifier was invented by DarkLight140 in a mini game where every role was a vanilla townie with some sort of role modifier attached.
Some moderators have attempted to create "Super Hated", "Ultra Hated", or outright "Fragile" modifiers that describe players that require two fewer votes to eliminate, three fewer votes to eliminate, or simply one or two votes to eliminate, but there is no standardization here.
While this does not appear except with the "Super Hated"-type modifiers, it is not standardized if zero or one is the minimum number of votes required to eliminate someone. For example, it is not clear whether someone actually has to vote a Super Hated Townie in order to eliminate them at three-player ELo (2 to eliminate).
The opposite is Loved, where it takes one extra vote to eliminate.
Use and Power
Hated players are easier to eliminate. It's a simple way to weaken a role, albeit one that's not necessarily fun for the player. However, Hated Townies can cause problems by causing ELo to occur earlier. If a seven-player ELo consists of three Townies, one Hated Townie, and three scum; the scum have already won if they know who the Hated Townie is. (This is not to say that the scum should simply endgame the Town in this case, as they may not be aware of one of the players being Hated.)
It is debatable whether it is better to not claim Hated Townie. By not claiming you are opening yourself up to getting run up to E-1 and thus unexpectedly eliminated; by claiming you are making yourself a policy elimination on the Day before ELo.