The software uses an HTTP request parameter to construct a pathname that should be within a restricted directory, but it does not properly neutralize sequences such as ".." that can resolve to a location that is outside of that directory. See http://cwe.mitre.org/data/definitions/23.html for more information.
FindBugs looks only for the most blatant, obvious cases of relative path traversal. If FindBugs found any, you almost certainly have more vulnerabilities that FindBugs doesn't report. If you are concerned about relative path traversal, you should seriously consider using a commercial static analysis or pen-testing tool.