they refer to Base Class Library of .NET
The Base Class Library (BCL) is a Common Language Infrastructure (CLI) standard library available to all CLI languages. CLI includes the BCL in order to encapsulate a large number of common functions, such as file reading and writing, graphic rendering, database interaction, and XML document manipulation, which makes the programmer's job easier. It is much larger in scope than standard libraries for most other languages, including C++, and is comparable in scope and coverage to the standard libraries of Java. The .NET Framework, being the first implementation of CLI, is the origin of the BCL. It is sometimes incorrectly referred to as the Framework Class Library (FCL), but the FCL is actually a superset including Microsoft specific namespaces.
The BCL is updated with each version of the .NET Framework.