Queues are used to limit traffic, or traffic shaping, or prioritize traffic.
You can use queues to configure CIR. To do that you need to specify
limit-at and max-limit in queue. Limit-at means that the traffic should never
go below that value (of course if bandwidth is available), and that is CIR.
You can have a better look at wiki
http://wiki.mikrotik.com/wiki/Manual:Queue