In a batch operating system, we group together same type of processes and schedule it for execution.
So, Lets consider there are two types of files (2 Fortran Programs, 3 COBOL programs) that need to be run on the CPU. Now , What Batch OS does is it groups all the files of similar type and executes it , then move on to the next "group" of files.
Hence, we first we take all the Fortran Programs (assumption) , and execute these, and next we take all the COBOL programs and executes it. By doing so , we need not to frequently switch between the compilers for different programming languages (Two in this case: 1 for Fortran , and 1 for COBOL).