Scaled takes smaller communities into account and bumps their posts up so they don’t get buried by posts from larger communities with more engagement.

Edit: here are the descriptions:

Active (default): Calculates a rank based on the score and time of the latest comment, with decay over time.
Hot: Like active, but uses time when the post was published.
Scaled: Like hot, but gives a boost to less active communities

https://join-lemmy.org/docs/users/03-votes-and-ranking.html