It a big hole in the ground, full of darkness and black and if anyone falls in they eventually come back up the top.
In "Bottomless Pit!" Grunkle Stan takes Dipper, Mabel, and Soos there to dispose of their unwanted items, but a strong wind blows all three of them into it when Stan stubbornly refuses to return to the refuge of the Mystery Shack. Each of them tells a story to pass the time as they fall, and they eventually come right back out of the top before Stan falls into it yet again and is falling during the credits.
In "Dreamscaperers" Mabel shoots the combination to Stan's safe out of Bill Cipher's hands with a nyarf gun into one of Stan's memories, and down into the bottomless pit to prevent Gideon from getting into the safe and stealing the deed to the Mystery Shack.