Origin
The "Brainrot Folder" trend emerged on social media platforms, particularly TikTok, where individuals approach strangers in public spaces and silently show them printed images of absurd memes (commonly referred to as "brainrot" content) from a folder or binder.
Overview
The trend involves a simple but effective formula:

A person compiles a collection of bizarre, nonsensical, or absurd memes into a physical folder
They approach unsuspecting individuals or groups in public settings
Without saying anything, they open the folder and display the printed memes
They record the reactions of the strangers

The appeal of the trend lies in the unexpected nature of the interaction and the genuine confusion or amusement it elicits from those being approached. In some variations, participants have expanded beyond folders to place memes on people's backs or in other unexpected locations.
Definition
"Brainrot" refers to internet content so absurd, nonsensical, or chaotic that it metaphorically "rots your brain." These memes often feature:

Surreal humor
Bizarre image edits
Random juxtapositions
Deep-fried or distorted images
Nonsensical text
Obscure references

A "Brainrot Folder" is simply a physical collection of these memes printed out and organized in a binder or folder for the purpose of showing them to strangers.
Spread
The trend gained popularity on TikTok, with videos of these interactions often garnering millions of views. The appeal comes from:

The genuine reactions of confused strangers
The awkwardness of the silent interaction
The contrast between digital meme culture being presented in a physical format
The shared understanding between creator and audience about the absurdity of the content