Skum are a race of grotesque, slimy, monstrous fish-men who serve the aboleths in the worlds of Dungeons & Dragons... no, not kuo-toa, the other slimy man-fish slaves to Lovecraftian fish-monsters. Yeah, they're kind of a redundant race.
The skum actually have a shorter history in D&D than you'd expect. In contrast to the kuo-toa, who showed up way back in the module D2: Shrine of the Kuo-toa in 1978, skum didn't appear until issue #67 of Polyhedron magazine, in January 1992 - 14 years later. They made it into first the revised Forgotten Realms campaign setting boxed set for Advanced Dungeons & Dragons 2e, and from there into the Monstrous Compendium Annual Volume One. They even made it into the Monster Manual for Dungeons & Dragons 3rd Edition... but they promptly slipped into obscurity from then.
The classic model for a skum is a piscine humanoid engineered by the aboleths from their stock of servitors - humanoids mutated by aboleth mucus into slimy, brainwashed, water-breathing creatures with translucent skin. The aboleths "improved" the genetic stock through fleshcrafting, engineering them into a more fish-like shape that is both more useful in their typically underwater environments and more appealing to their sense of aesthetics. They serve their creators as general beasts of burden, laboring on their behalf to do all the things that a giant (and lazy) fish can't really do.
The AD&D version has far more lore tied to it, covering such things as reproduction and what features the aboleths actually removed or added. It also describes them as having been bred to be coldly logical and free of emotions, but also of sharply limited intellect; so they're kind of dumb, but the things they do always make logical sense, albeit to a moron. They also have a telepathy that only works with aboleths and fellow skum, and are extremely susceptible to mental control.
In Pathfinder, Paizo decided to double-down on the Deep One similarities to skum. Originally called the Ulat-kini, skum are the fallen remnants of a race of all-male man-fish engineered from human slaves through aboleth fleshcrafting. They were once the most ubiquitous and prized of the aboleth's slave-races, but as the aboleths fell into decline, the skum were mostly abandoned, and now generally eke out a degenerate living as primitive tribals in various watery regions of the world - ironically, these "masterless" skum actively hate their former gods, and will refuse to serve them again. In contrast, in the Sightless Sea of Orv in the Darklands, aboleths still retain command over civilized skum, and have even bred specialized subspecies of skum to do their bidding.
The skum of Golarion are biologically immortal, but their lack of female skum means they must mate with human women - either bribed with treasure or simply raped - to propagate their species. Their sons are always either skum, or disfigured humans who transform into skum as they age.
Whilst they were a big deal in the early days of Pathfinder, they've kind of been overshadowed ever since actual Deep Ones became a monster.
Dungeons & Dragons 4th Edition dropped skum entirely; they already had aboleths being served by kuo-toa (aka "crazy, slimy, Lovecraftian fish-men things") and Abolethic Servitors (once-humans turned into slimy, amphibious, translucent-skinned brainwashed horrors - they've been around since AD&D, but this was the first edition where they had an actual statblock), so skum simply felt redundant. What did they add to the game that the other two monsters didn't?
But... D&D never was one to throw away a good monster name. That's how we wound up with the Thoul. So, in the Monster Manual 2, the 4e list of Oozes was expanded with the Abolethic Skum; the result of the ritual to transform a humanoid into an Abolethic Servitor going awry and dissolving the victim into a semi-sapient slick of snot. In this, they kind of also filled the role of the Shaboath, an ooze-like golem made by aboleths from their own mucus.
Skum finally made their debut in Dungeons & Dragons 5th Edition with the 2019 adventure module Ghosts of Saltmarsh. They gained a really creepy new look that combines element of human, octopus and jellyfish, finally making them at least look different to the kuo-toa. In this edition, they are described as victims of a mutagenic plague carried by aboleths, which transforms humanoids into horrible aquatic monsters and also remolds their minds so they fall under the aboleth's psychic thrall irreversibly. These skum can't leave the water for more than brief intervals - after 10 minutes out of the water, they start dissolving into a noxious slime - and so carry out the will of their masters underwater; surface work is left to Deep Scions instead.