Like the Windows Insider program, the Office Insider program is an ongoing preview program for Office 365 subscribers (both Windows and Mac), providing early access to the latest Office features. It's open to anyone who has an Office 365 subscription (administrators need to enable First Release on business accounts before business users can install the Insider Builds).
Insider Slow is the original Insider program. Users in this ring will receive an update about a week before the official production release each month, and then the official production release when it goes out (2 updates per month). These two are generally very close in terms of features and fixes.
Insider Fast is a new level that delivers weekly updates from the development branch. There will be features available to users in the Insider Fast ring that won't be available to Insider Slow or Production. Because this build is from the development branch, these users always be a version ahead of the official production release.
You can check to see which update channel you are currently using in File, Office Account (or Account in Word or Excel). The update channel is listed below the suite name and above the version number.
Join Office 2016 for Windows Insider
If you'd like to join the Office Insider program and have a consumer subscription (Home, Personal, or University subscription) go to the Additional Install Options page of My Account and choose an Insider option from the Version drop-down menu.
Office 365 business and enterprise users get the Insider Build through First Release program. To switch from Insider Slow to Insider Fast, you'll need to set the following registry value:
Don't want to edit the registry? Download and run this prepared .reg file. (Right click on the link and choose Save link.) InsiderFast
Join Office 2016 for Mac Insider
To join the Office for Mac insider program, open an Office 2016 for Mac application then click Help, Check for Updates. In the Microsoft AutoUpdate dialog box, select Join the Office Insider program to get early access to new releases check box and choose between Fast or Slow updates.
The iOS, Android and Windows Mobile apps (Word, Excel, PowerPoint) are also available to "Insiders".
More information on the Insider program is available at Be an Office Insider
More information on the Insider program for Office for Mac is available at Welcome to the Office Insider for Mac community!
Overview of update channels for Office 365 ProPlus (TechNet)