Basically an incantation for a childhood horror tale.

Saying "Bloody Mary" 3x will call upon the spirit of the queen who bathe in the blood of youth (well, one variant of it, think Beetlejuice if you watched it before)

For more info: https://knowyourmeme.com/memes/bloody-mary

A child's... game? Kids often at school or sleep overs will spread and test the rumor of summoning Bloody Mary. In a dark room, while looking at your reflection, you say "Bloody Mary," three times.

The low light of the room and the human brain's tendency to use short cuts makes your eyes perceive a distorted reflection of yourself after you stare at the reflection for a while.