A value mapping is a component that associates the value accessor used to read a value from a source object with the value accessor used to write a value to a target object.
An example of a value mapping is one that specifies how to read the description of a product from a PIM (product information system) and write that description to a field on a Sitecore item that represents the product.
Data Exchange Framework includes a default value mapping. In most cases, this value mapping will be sufficient for any value mapping you configure. It is unlikely that you will need to extend the default value mapping, or to implement your own.