I've had this message before in the recent achievments reputation... box. I'm guessing that it's because someone has more than one account with philosophy stackexchange?
Serial voting means that one person has up- or downvoted a large number of your posts in a short period of time.
Serial upvoting occurs when a user appreciates your work and wants to give you a lot of reputation in return. It also occurs when two users have agreed to upvote the posts of the other, or when a user creates a 'sock puppet' account, just to upvote the posts of his main account.
Serial downvoting occurs when a user has a grudge against you.
These kind of reputation changes are reverted because votes are meant to reflect the content of a post, not the appreciation of the author.
You don't need to worry about this message: in all but a few cases this is someone else's fault - and when it is your fault, you know it.