Learn JCL: In The Simple Way.

Job Control Language (JCL) is the command language of Multiple Virtual Storage
(MVS), which is the commonly used Operating System in the IBM Mainframe
computers. JCL identifies the program to be executed, the inputs that are
required and the location of the input/output, and informs the Operating System
through Job control Statements.
In mainframe environment, programs can be executed in batch and online
modes. JCL is used for submitting a program for execution in batch mode.