It's a tiny "plug-in" that you install in Chrome (don't worry — it's easy.) Then, once it's installed, it can let you know if you need to change your account password.
"If we detect that a username and password on a site you use is one of over 4 billion credentials that we know have been compromised, the extension will trigger an automatic warning and suggest that you change your password," Google said.
It's part of Google's Password Checkup tool. Here's how to install it.
Now, if you're browsing the web and the plug-in detects that you're using a password it knows has been compromised, it will show you a warning that looks like this, alerting you to change your passwords.