Plim is a planet that is home to the species of the same name. From the outside, it appears to be barren and lifeless, however this is because of a holographic field that surrounds the planet.
This same field also shorts out the electronics of anything passing through it, including ships; as a result of this, the Plim have been left stranded on their planet.
There is at least one major city on Plim, which also has the same name as the planet and its inhabitants, composed of hive buildings. Society on Plim is entirely automated, with all the needs of the locals taken care of by machines. Most prominently among these is the television-like Feed, which presents programs generated by computer algorithms based on what the Plim say they like.
The most popular of these is a "prophecy" which says that the Plim don't have to try and take down the field, because one day a hero will fall from the sky and do it for them. Part of this comes true when Zim and Dib crash on Plim during a chase in space; since the Plim can't decide which of them is the hero, they're given a week to prove which of them it is.
The Plim appear to have once been more violent than they are now, as there is a vault on the planet containing weapons that the ancient Plim locked away. The prophesied hero would have access to this vault.
In Issue 43, Dib goes to the archive that contains all of the records kept by the planet's automated systems and discovers that the Plim's ancestors built the planetary field, and using the controls they left behind shuts it off. However, he then realizes to his horror that the field was meant to hide Plim from the Irken Empire, who quickly arrive and conquer the planet. The Plim themselves don't care, as being enslaved is no different from their reliance on the Feed.
When Dib tries to steal the ancient weapons from the vault before the Irkens can, he learns to his surprise that there were never any weapons. Instead, the vault contains archived Feed programs, meaning that the Plim probably never were that much different from their current selves.