This occurs when using the 'preview' option to send a preview of an email in the editor screen. When you send a preview email to yourself you will see the custom field place holders. For example, if you have inserted the variables for First-Name and Last-name, the preview email might display the following salutation in the email:

Hi %%First-Name%% %%Last-Name%%,

This occurs because, when previewing, there is no contact list and accompanying values for custom fields associated with the email when you send the preview. The place holders simply show you where data will be located. Note that dynamically created links such as the Unsubscribe Link, Forward to a Friend Link and 'Web Version of this Email' links also will not contain an active link if you send a preview campaign from the 'preview' option in the editor screen.

All fields that are dynamically created at campaign send time will use placeholders .  This includes the %%unsubscribelink%%, the %%webversionlink%%, and any links to custom forms.

If you would like to see the data instead of the place holders, add yourself to the contact list and send an email to yourself as a test. Or alternatively maintain a contact list with records that include your email and values for the corresponding custom fields, and send the preview to this list.