The mobile label for a field can be set explicitly to a text value, or via a formula. When the formula points to another field, this field’s value is used as the mobile label.
Referring to the image above:
This is a label’s Field ID; this is the variable that can used to reference the label.
The label’s value is "Plan #".
Another field’s Mobile Label references the above label using the Field ID.
The label’s value for the referenced field is used as the label in the mobile layout.