Inside Apex Forms, the elements that comprise a form (fields, sections, etc...) have labels that are user friendly, but they also need a way to be uniquely identified by the system.  That's what the Binding provides.  You'll find the Binding value on the details of the form elements under the information tab. 

When naming your fields or sections, you will find there are three related properties that you need to consider. The Binding is the name used by the Information Tree to reference the element.  The Label is the text that will appear on the forms when users are populating the form.  The Name is the text that identifies the element on the form design page.

Changing the Binding value immediately updates the Information Tree with the new value, and vice-versa. This makes it easy to test forms with read-only fields by populating the test data in the information tree.

You need to pay close attention to the Binding because it is the value used in validation. If you accidentally reference the Name or Label, your validations will not work.