I found this in the "Users" listing
What is negative reputation?
The profile for this user (who was last here 3 years ago) shows reputation 101.
When looking at users tab you can choose what to display - reputation, editors, voters, new users, moderators. And you can also use period on which you want to concentrate. (I am not really sure which period is chosen by default if you do not change anything.)
If you display reputation for week, month, etc., it is displayed as negative if the reputation change of that user was negative in a given week/month. You can check users with username containing "banach" and their reputation change for the current week, month, quarter, year and all time. (The last one shows reputation of the users on the site. The other tabs correspond to reputation change in the chosen period.)