It's the little popup box you see when you type in a password to a website that requires a login and Chrome detects that the password has been compromised. At that point, you'll get a helpful box asking if you'd like Chrome to check all of the other passwords you've saved into the browser.
It's a good feature and it will definitely help keep your passwords more secure.
Even better, Google has now rolled that feature out to Android users. It's the same idea, but with a slightly different implementation. In this case, the check will occur when you log onto a site from your Android device using a password stored in the Android OS. The OS will perform a quick check of the password in question to see if it gets flagged, by searching through its constantly updated database of passwords that have been impacted by a data breach. It will let you know if your username and password came up in its search.
Just like Google Chrome does, it will then offer to check all of the passwords you've stored in the Android OS. It's a good feature in Chrome and a welcome addition to the Android OS. Best of all, it creates a more seamless user experience as you hope from your favorite Android device and back to your PC.
If you're interested in checking it out, and you're using Android 9+, just open your phone's Settings App, tap "System," "Languages & Input" and then "Advanced."
Once there, tap the Autofill service, and then tap Google to make sure the setting is enabled. Follow those simple steps and you'll increase your password security on your phone. Kudos to Google for the addition.