An idiot plot is when one or more characters become totally clueless about things they normally could figure out quickly, so that the plot only works if/because the characters are being idiots. It is used by writers as a careless and/or lazy way to move the plot forward.
Many fans count idiot plots among their least favourite aspects of canon. However, for some fans, an idiot plot can provide an opportunity to explain or fill in the blanks via fiction or vids.
Fanfiction can be just as guilty of employing the idiot plot as the canon they are transforming.
Some Brief Examples in Canon
Examples Wanted: Editors are encouraged to add more examples or a wider variety of examples.
Angels & Demons has a gaping hole that not a single character sees. The book emphasizes the undetectability of the antimatter containment, and the protagonists go as far as to wipe out power to entire city blocks so that they can look for it without any background noise. But no one thinks to look for the camera pointed at the containment device that is actively broadcasting video. If there's video, there's a signal. If there's a signal, it can be triangulated. The movie does address it when Langdon lampshades how dumb everyone has to be to let things like that happen, but they still happen nonetheless.
Some Examples in Fandom
Examples Wanted: Editors are encouraged to add more examples or a wider variety of examples.
My Immortal has a lot of this going on, but a particularly strange one is the Triang Relations at the story's core. Ebony loves Draco, but she also loves Vampire (Harry) Potter—standard love-triangle nonsense, right? Except the story also reveals that Draco and Vampire are bisexual and love each other as well. Despite this, nobody in the story ever suggests going the One True Threesome route.