The errors report shows you which error messages were seen by your website visitors. Xlogan will give you the error number and description, and tell you on which page the error occurred. It will also tell you the referrer, the times that the error was seen, and how many times.
It is common to find a lot of HTTP 404 "File not found" errors, though in these cases you should check that the Referrer displayed is not your own site - if it is, that means you have a broken link somewhere, which you should fix.
You should also check for any HTTP 500 "Internal Server Error" messages, as if visitors are seeing these it means you have a broken script - again, something which needs to be fixed.