A facilitator is the person who supports the group processes during the meetings. The facilitator role is separate from the leader role (who holds the operations of the circle). A facilitator is only the one preparing an agenda draft, starting rounds etc. In a well-trained group, the facilitator is just as aware of processes as the other circle members, and rounds flow naturally.
A facilitator ensures equivalence of voice by facilitating rounds and other processes. The facilitator (or anyone in the group) can be one reflecting back what someone has said in order to make sure it is fully understood. Having a facilitator means the group can relax into the process which makes for calmer, more focused meetings.