From the field mapping configuration page you'll see the following button:
Click on this to add a new field map row:
You can enter any text here, and that text will be sent exactly "as is" to the selected integration field. One useage of this feature is the ability to specify a unique page identifier (to supplement the Page ID) to differentiate between leads from different pages.
That's fine and dandy, but the real power of custom fields comes from the ability to combine form fields together.
Including Form Fields
You can include the value of a form field inside your custom field by specifying a form field name in parentheses:
Assuming you have a form field with the name "First Name", then this value will be substituted directly in during a form submission.
For instance, if you have the following form submission:
And the following custom field:
When the above form submission is handled by Unbounce,"Aaron is super cool" (without the quotes) will be sent in place of the custom field you just specified.
Combining Form Fields
Since custom fields are simply text strings, you can include as many form fields as you like:
As long as the name in parentheses matches exactly with a form field, then the form field value will be substituted into the custom field during a form submission.