Anyone understand what this means?

Specifically, what class library are they refering to, who writes it, in
what language, and how does it work? Why would anyone want to use a
different I/O library?

From Microsoft's white paper:

File I/O
Visual Basic 6.0
File I/O statements are included in the language.
Visual Basic.NET
File I/O operations are available through class libraries. Removing the
file I/O statements from the language allows different I/O libraries to be
easily used from Visual Basic.NET. This would be more awkward if the file
I/O statements were in the language, since identifiers like Open, Close,
Print, and Write would be reserved words.
Upgrade
Wizard The file I/O statements are upgraded to the corresponding functions.
For example, the following code:
Open "MyFile.txt" For Input As #1
is upgraded to:
FileOpen( 1, "MyFile.txt", OpenMode.Input )