Beginner question: What is binding?

If you are talking more about databinding, then I'd say the definition used by Microsoft in their Data Binding Overview works quite well:

"Data binding is the process that establishes a connection between the application UI and business logic. If the binding has the correct settings and the data provides the proper notifications, then, when the data changes its value, the elements that are bound to the data reflect changes automatically. Data binding can also mean that if an outer representation of the data in an element changes, then the underlying data can be automatically updated to reflect the change. For example, if the user edits the value in a TextBox element, the underlying data value is automatically updated to reflect that change."

In other words, it's a way of getting data from a data-source (such as a database or XML file) and applying that data to an interface or display element. For instance, a form might have a drop-down list of values that are stored in a table within a database. Data-binding is the process of 'binding' the values from the database to the list. One-way data-binding is usually 'read-only' but two-way data-binding allows the user to update the values back to the underlying data-source.