Summit

I was going to point to the release notes but then I realized I didn't released them yet 😆. Yeah it's what everyone else said. It's the number of times you upvoted a post/comment by that user subtract the times you downvoted them. It's local to your device though so if you vote on web, it will not be updated on mobile.

You can turn it off in Settings > Misc > Show per-user scores.

Six

Wonderful day!

I've never used the wonderful client yet, but was curious too, and checked the source code.
Evidently, it should be "person's score":

// ...
val personScore = personTracker.getPersonScore(targetPersonId = personId)
if (personScore != 0) {
  append(" [")
  val s = length
  append(personScore.toString())
  val e = length
  append("]")
// ...

Source: LemmyHeaderHelper.kt

Seems to be the upvotes - downvotes that you've given a person across their posts.

If you look at the "WIP 1.82" post, you can find something related to user scores. I'm not entirely sure how it's calculated, but I think the intent is to show how many up- and downvotes a user gets.