Nobody wants a 404 on their site, but they happen, and the bigger your website the more likely they are.
A HTTP 404 is a status code returned by the server when a page or asset (image, stylesheet, script, etc) doesn't exist. It may have existed at some point, but now it doesn't.
How does that happen?
Human error, probably. Pages get deleted. Assets get moved or renamed. There could even be a typo in the original link taking you to a page that never technically existed in the first place. Either way, your user is sat on a page that doesn't exist and they've hit a dead end.
Janitor will crawl your site and check all the links it finds for 404s. When Janitor finds a broken link it will inform you via your selected channels (Slack, email, etc). Don't worry, we won't bombard you with messages with every 404 found, we'll send you a digest after the crawl has completed.
To help find the problem links we'll give you a useful list of all of the pages that contain the links, along with the pages that actually 404. With that information it should be pretty straightforward to pinpoint the issues and get them fixed quickly.