It's a built-in feature of Gmail, rather than a Labs feature or an installable addon - once the gradual rollout reaches your account, it will just appear in you

It's a built-in feature of Gmail, rather than a Labs feature or an installable addon - once the gradual rollout reaches your account, it will just appear in your Gmail sidebar (or, if you're already using Rapportive, in the Rapportive info). Google is doing a characteristically excellent job of communicating this to users.