Release is a common timebox across projects that helps the release manager to drive a deployment schedule to production. Release has smaller time boxes called Release Versions which specify fine granular time units like sprints. The changes in the release and release versions can be seen in the history section. You can also assign release versions to items from the detail view of items. For mass assignment of a release version to multiple items, use the tab Mass Edit.
A project can only be assigned one release. A release can include multiple release versions.
For more information you can check this blogpost.