Mainframe is high performance computer mainly used for large data computing.
Mainly financial sectors use it. Also companies which are involved in medicine, health and insurance use it.
IBM has its setup and consultancy companies like Infosys, Cognizant, Accenture, Capgemini etc use it and pay IBM as per the usage.
A software engineer must be pro in domains like COBOL(common business oriented Language), JCL, CICS