A Data access layer is a module of computer programming logic that provides simplified access to data stored in persistent storage of some kind - usually a entity-relational database.
This Data Access Layer is used in turn by other program modules to access and manipulate the data within the data store without having to deal with the complexities inherent in this access.
For example - the DAL could return a reference to an object (in terms of object-oriented programming) complete with it's attributes instead of a row of fields from a database table. This allows the client (or using) modules to be created with a higher level of abstraction.