What is CICS in one paragraph? Why do we need it?

I get batch COBOL + JCL. CICS is still a fog. Is it the thing that makes green screens work?

CICS is the online transaction layer. User hits a transaction id, CICS runs your COBOL program, talks to files/DB2, sends a map (screen) back. Short-lived tasks, many users at once.

Batch = overnight jobs. CICS = daytime clicky / terminal work.

Maps are the screen layouts (BMS). That part confused me until I saw one painted in the map editor.