Office 365 First Release is a fairly new feature that provides customers the opportunity to get a small selection of significant enhancements to the Office 365 user experience, SharePoint Online, and Exchange Online first. If you opt-in to First Release, you will receive new qualifying feature updates first, a minimum of two weeks before customers in our standard release group.
Updates to Lync Online, Exchange Online Protection, Office 365 ProPlus, or other related services are not included in the First Release program at this time.
By default, all Office 365 customers are part of the standard release group. If you’re in this group, you will receive the new feature updates a minimum of two weeks after availability in First Release. The vast majority of service updates will be made available to all customers via our standard release processes.
If you are interested in opting into the release group, an Office 365 admin can log into the admin portal and select First Release under service settings.
To learn more about this, check out the following blog post