Brainrot is a word that some might use when referring to constantly thinking about a topic, person, place, or thing. It's a spoof word that's relatively close to the definition of a hyperfixation. Both words stand for the same meaning, however, brainrot isn't meant to be taken as literally. There aren't any brains rotting, here!
Person 1: Hey, man. Are you alright? You've been going on about Gon Freecss for a while, now..
Person 2: I'm alright! I just have a case of brainrot, is all. NE, NE KILLUA!
Brainrot Content refers to the various "short" type-videos (typically a YouTube short, a TikTok, an Instagram reel etc.) that swarm Social Media feeds. It's very easy to get sucked in and watch one short after another without even noticing. This lack of stimulation (or more correctly, EMPTY stimulation) "rots" the brain because of its self-repetition and low quality.