A value accessor is a component that associates a value reader with a value writer.
An example of a value accessor is a value accessor used to read or write the a field on a Sitecore item that represents a product.
A value accessor is like a .NET property. A value reader is like a property getter. A value writer is like a property setter.
Data Exchange Framework includes a default value accessor. In most cases, this value accessor will be sufficient for any value accessor you configure. It is unlikely that you will need to extend the default value accessor, or to implement your own.