Underswap is one of the many Alternate Universes, or AUs, of Undertale, as well as one of the most famous ones. In this AU, as indicated in the name, every characters are swapped with their counterpart, or what the developper of the AU thinks as their opposit. For example, Sans is swapped with Papyrus, which means Sans is the hopeful, active, positive, "human-hunting FANATIC" sentry of Snowdin, while Papyrus takes the role of the lazy, protective brother, who will also be the judge and the sleepy sentry.
There are some features that are either added that didn't exist in Undertale, like the fact that Papyrus smokes, or some others that are kept from Undertale, like the fact that Papyrus stays recultant to kill you even in the genocide run.
At the moment, not every characters have a swapped-version of themselve in Underswap, so a lot of suggestions are made from the fandom, like Gaster being swapped with River Person, or Froggit being swapped with Whimsum. Therefore, the game isn't out yet, though there's still a demo which gives CANNON swaps, like for instance Papyrus and Sans, or for example Undyne and Alphys, Undyne being the "nerdy", shy, anime-fanatic but really nice Royal Scientist, and Alphys being the brave, dangerous and (dangerously) friendly chief of the Royal Guard, or even the protagonist : Chara is swapped with Frisk, but the behavior isn't cannonly showed. People tend to believe that Frisk isn't a genocidal spirit but a true pacifist spirit (which is shown in an Unitale Genocide Papyrus fanmade fight : Chara, after killing Toriel and Temmie (who is swapped with Flowey), says "It's me, Chara", which would lead to believe that Frisk's spirit only appears on the True Pacifist route), while some other want to believe the swap stuff goes far enough to make Frisk genocidal.