Similar to a HTTP 401, a 403 code states that the browser sent some auth details, but they were incorrect, and it shouldn't try again with the same details.
What can cause a HTTP 403 code?
HTTP 403 statuses are far less likely to happen that other error codes like a HTTP 401 or a HTTP 500, this is because a 403 normally requires the browser to have sent some credentials with it that were incorrect.
However, it's entirely possible that a misconfigured server or application can cause this status to be returned, especially in larger more complex applications.
What is checked by Janitor?
Janitor will run through your website, crawling it page by page. If a 403 is found then we'll make a note of the page it was linked from (how we found it) and the page the 403 was found on. These details should be enough to pinpoint the problem