This opt-in program updates your Chromecast to the latest firmware prior to the wide release, but these updates are said to be of the same quality as production updates. Members of the program can expect their updates to arrive shortly before the full push of the official production firmwares.
In essence, joining the program gives you a word in the last say before the latest update hits all Chromecast devices, as Google is looking for feedback. Feedback can be submitted from iOS, Android, or your computer. Steps for each are listed at Share feedback about Chromecast - Chromecast Help
All Chromecast devices (1st and 2nd gen, Audio) are eligible for the program.
If you're interested, visit this link for FAQs and more info: Chromecast Preview Program overview - Chromecast Help or follow the steps below to jump right in (from the FAQ page).
- From your Android or iOS device, open the Google Cast app Chromecast cast icon.
- Tap the DEVICES tab to see your available Chromecast devices.
- Scroll to find the device card for the Chromecast you'd like to enroll in the Preview Program.
- In the top right corner of the device card, tap the device card menu .
- Tap Device Settings.
- Tap Preview Program. If you don’t see Preview Program, we aren’t accepting new members at this time. Please continue to check back as opportunities become available.
- Choose whether to receive email notifications by moving the slider to the right or left. Note: We recommend allowing email notifications so you’ll know when new updates are pushed to your Chromecast device.
- If you opt-in to email notifications, you'll be asked to sign into your Google account if you haven’t already.
- Review the contents of that page, and tap Join Program.
- Review the contents of the page, then tap OK, GOT IT.
Thanks, @CJM for the tip!