Caldera Forms has two types of variables, passback and static. Passback variables are used to send data to the next HTTP request as query variables when doing a redirect. Static variables are used to as containers to move data inside of the same submission.
Variables values are generally set using Caldera Forms magic tags.
The primary uses for a static variable is to copy one or more field values and make the result available to processor settings. For example, if you wanted to pass the values of one or more checkbox fields to a MailChimp custom field using the Caldera Forms MailChimp add-on. To do this, you would create a static variable, whose value was the magic tag for those fields. Then you could set the MailChimp field value to that static variable.