I know this user have more than 1K rep, but today I checked his reputation, he got negative reputation and also it mentioned Serial upvoting reversed. What it means and for what reason he got negative rep?
It means the user got votes from the same person, or from a group of people, in a way that they were considered serial voting: votes given to posts not because their quality, but because the person who wrote them. To make an example, I could up-vote the questions asked from my girlfriend, but if I start voting almost of the questions she asks, and the number of votes is higher than a prefixed number of votes, then they would be reversed.
The reputation change is negative because the reputation obtained from serial up-voting is removed. If it were serial down-voting, the users would get back the reputation they lost, and in that case the reputation change would be positive.