A 'disposition review' is where an admin reviews the items that have reached the end of their retention period to decide ...

A 'disposition review' is where an admin reviews the items that have reached the end of their retention period to decide whether or not they can be safely deleted ('disposed').

When items reach the end of the retention period, reviewers will receive an email notification and can go to the Dispositions page to review the items.
Learn more about disposition reviews

Note:At this time, this setting applies only to content in SharePoint, OneDrive, and sites for Office 365 groups. Disposition reviews don't include content in Exchange, Skype, public folders, or mailboxes for Office 365 groups. If a label that triggers a disposition review is applied to content in those locations, that content will be deleted automatically at the end of the retention period and won't be available for review on the Dispositions page.