There is a link to the manuals at the top of this page. If you follow that link and look into the JCL Language Reference manual, navigate to section 2.1, you can find this:
Jobs placed in a series and entered through one input device form an input stream. The operating system reads an input stream into the computer from an input/output (I/O) device or an internal reader. The input device can be a card reader, a magnetic tape device, a terminal, or a direct access device. An internal reader is a buffer that is read from a program into the system as though it were an input stream.