In Agile methodology, a release represent a fixed time period, at the end of which a potentially a shippable product is released to be used by customer/end users. It is different from a sprint or iteration which is also a set time period to create working software at system level, which might not yet be ready to be used by customer. One release may contain 3 or more iterations and spans on months.
From the Agile point of view, a release plan provides a common delivery schedule that helps to convey the timeline vision of the product owner and management to the team and other stakeholders. By dividing delivery of the features into different releases help management and the team to manage the over all project schedule.
Release planning is performed before every release, product owner and team members take part in the release planning meeting. The user stories, epics or features that need to be completed are moved from product backlog to the release (backlog). Items are selected, prioritized and estimated based on the vision of the product managers.
A standard tool to measure and monitor the progress of the release is Release Burn-down chart. Product owner can setup a weekly meeting with the team to see the actual vs planned progress of the release.