what is TSR and what is use of it?

TSR (Terminate-and-Stay-Resident)

A utility program that loads and remains in random access memory (RAM) even when not running, in order to be available instantly when required.TSR programs were popular as extensions to DOS, but largely have been replaced by more sophisticated modules in more recent operating systems (OSs.)

Calendars, calculators, spell checkers, thesauruses, and notepads are often set up as TSRs so that you can instantly access them from within another program. TSRs are sometimes called pop-up programs because they can pop up in applications.

TSRs are unnecessary with multitasking operating systems such as Windows, OS/2, and the Mac OS.