No docs neither time to write something decent, as far as I remember:
FetchTimeout it's a sort of fallback for fetching jobs. This software use redis pub/sub to be notified when a new job has been enqueued. Redis pub/sub however doesn't guarantee delivery so, should the RedisConnection loose a message, within the FetchTimeout the queue will be scanned frmo scratch and any waiting jobs will be processed.
InvisibilityTimeout it's part of mechanis to requeue the job if the server processing it died for some reason (look at FetchedJobsWatcher)
ExpiryCheckInterval, IIRC, simply dictate the time that should pass within a check of Expired jobs and the next one. There's a thread running as a IServerComponent that take care of deleting expired jobs from redis (remember that the redis keys of this software are set to never expires)
For example, what's the meaning of
FetchTimeout
/InvisibilityTimeout
/ExpiryCheckInterval