That's an incredibly generic term. An abstraction layer is a layer of interface that abstracts a more specific or more complicated interface below it from whatever uses it.
An example is a database abstraction layer that provides a standard interface to many different databases hiding the specifics of each database from and providing a uniform API to the application using it.
Apache provides a protocol abstraction layer to ease the development of modules to provide protocol implemenations without having write a special interface for eash. This, however, is usually only of interest to developers.