Good to check if you have waiting messages in the queue

The Warning and Critical value is the depth of the queue or better said the number of messages that are present in the queue. so having more messages in the queue then the warning will give you a warning. Same for Critical.

so putting Warning on 2 and Critical on 4. Then you will get a warning if you have 2 messages in the queue and a critical warning if you have 4 messages in the queue.