Isolation levels.

Two people edit the same document at once. The database has to decide how much each sees of the other’s half-typed work.

Loosest setting: you read a sentence the other person hasn’t saved — then they delete it. That’s a dirty read.

Turn the dial up to read committed: you only ever see saved work, never anyone’s drafts.

Higher still — repeatable read: a line you’ve read stays put for the rest of your turn, even if someone edits it.

But brand-new lines can still appear between two reads of the same section. Those are phantoms.

Top of the dial — serializable: it acts as if you each took the whole document in turn. Spotless, but everyone waits more.