Well, it can be a failry big subject, but I'll try to give a brief overview. Basically binding is how fields in the form are linked to nodes within the data DOM (so where data values will be imported from, and subsequently exported to).
Normal binding: The default. This means the data DOM and the form template DOM will follow the same hierarchy. So the field formName.subformName.fieldName will be bound to the node in the data at formName.subformName.fieldName.
Explicit binding: You can specify an explicit SOM expression that points to the node in the data DOM that your field binds to. The advantage is that the data DOM and form template DOM don't need to share the same hierarchy.
None binding: The field is not bound to anything in the data DOM, no data will be imported/exported for this field. It's rarely used, but there may be a case where you want a field on a form, but don't really care about the value afterwards.
Global binding: All fields with the same name on the form will share the same value. So if you have ten fields named MyField on your form, and typed in a value in one of them, the value would appear in all 10.
Hope that clears things up a bit.
Chris
Adobe Enterprise Developer Support