Once Security has been applied to your profile and you have selected a member database, the currently logged in user's information will be available anywhere that the wildcard menu is shown. You can use these values to show messages like "Hello [currentUser.firstname], welcome back" etc.
Using CurrentUser Variables to Pre-Populate Forms
Another common use of the currentUser wildcards is to pass them into a Form (when linking to a form) to pre-populate the Form. In this way, you can track the new submissions as belonging to the logged in user.
For example, if you are using your customer's database as the member database and have a link to a form that allows customers to place a new order, you can pass the [CurrentUser.record_id] into a hidden field on the Order Form to track that the order was placed by the currently logged in user.