From This Might Be A Wiki

Songs that make use of microtones - intervals besides semitones, or notes besides the 12 tones of western music.

John Linnell began using microtones more in the 2018 Dial-A-Song releases, but there are also several songs on House Of Mayors that make use of them. They feature on a couple experimental tracks from the early days as well; John Flansburgh has spoken about the band he and Linnell briefly had with upright bassist Dave Lindsay before They Might Be Giants:

Dave and John were really intensely interested in microtonal music and I really felt like I wasn't up for that. I just didn't have enough musical experience to start fooling around with microtonality, which is a really complicated and kind of odd thing.[1]