Hi,
TSR stands for Terminate and Stay Resistant. A TSR program installs itself (or gets installed) and will stay permanent in the system even if the program returns from its main routine.

The clock which we will develop here is an excellent example of a TSR program. We want a clock which outputs its time on the statusline, but we want also to use the calculator's commandline as usual. A program which loops within its main routine (like the one developed in tutorial S1P2) cannot be used for this purpose. The clock program has to return from its main routine letting the system think that it has finished.