A Persistent Instance is one that does not release when the last player leaves it. Its state is preserved for a number of days and, if anyone in the original party re-enters that instance, the instance is restored to its prior state. This means that a raid no longer needs to complete an instance in a single, marathon session. They can set a time limit for the first session and meet again the next day to continue where they left off. This will also remove the issue of a server reset during a raid. More information on Persistent Instances can be found here: