The term "Boss" has two distinct definitions in the game.
"Boss" monsters are a generic MMO term for special monsters encountered within dungeons, instances, or sometimes in the open world at special points. They called "bosses" because they traditionally have higher stats or levels than the monsters around them, or because they sit on specific choke-points that make them appear to be in charge of that area.
TSW has taken that a step further and made it a distinct difficulty rating rather than just a generic term. A "Boss Difficulty" monster in TSW is indicated by a Skull icon next to it's name.
These are typically named monsters and often quest objectives (indicated by also having a flag next to their name). They are usually harder than Normal monsters, but not by much.