CRM is an area within Admin where you can manage Users that sign up to your Site, or fill in Forms creating Cases; and these associated Cases.
Clicking into a User enables you to see when this User first registered; email addresses; ability to send a password reset to this User; which Secure Zone (if any) that they are assigned to; Cases - Forms that they have filled in on your Site; and Custom Field Sets that are attached to each Users.
Where can you find CRM?
While viewing your Site Admin, use the left-hand menu to click "CRM" and expand the sub-menu. Next click on "Users" to view a list of Users on your Site.