CRM Sync allows you to automatically BCC your emails to your CRM system. Our help center contains instructions for configuring CRM Sync.
If the CRM Checkbox is not allowing you to click it to change the status, check that it is enabled in the Extension Settings by clicking the Bananatag button at the top of the page, and then selecting Extension Settings:
Ensure that CRM Integration is set to Enabled:
If it is already enabled, try disabling it and re-enabling this option. Once Enabled, you should be able to click the CRM box in your messages.