In the notifications bar there are some notifications with the green plus sign and a number, for example:
What do they mean? E.g "+2"
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This is the net reputation change due to the event raising the notification.
Positive change would cause the number to be green, negative change would cause a red color.
Some common examples for positive rep change: upvotes you get on your posts, answer you posted got accepted, edit you suggested was approved, etc.
Some common examples for negative rep change: downvotes you get on your posts, user was removed, bounty you started, etc.