The JIRA integration allows to select a commit message (including JIRA key) directly from (open) JIRA issues and to optionally mark issues as resolved on Commit.
The JIRA integration is only available for the Professional edition and will only be present, if the Bugtraq properties have been set up properly for your JIRA server.
When connecting to a cloud-based JIRA instance (*.atlassian.net), you have to login with your username, not your email address. You can find your username in your Profile (top-right corner).
The commit message selection is available in the Commit dialog.