Prorogue is a technical way of saying "put off" or "delay." Usually, it describes something a government body does: when a congress or parliament prorogues, it's putting off business for a while.
Prorogue comes from the Latin word prorogare meaning "to stretch out." When you stop a meeting and decide to meet again at a later time, this "stretches out" the work being done. Remember that prorogue is close to prolong, which means pretty much the same thing, but without a break.