The MASSIVE warning in caps has me worried

It's a term from the old DOS games, where one could write a program that does things like the following:

1. Hooks into the keyboard interrupt. The new interrupt handler (ISR) logs what key has been pressed to a file (without consuming it) and then jumps to the old ISR.

2. Terminates, but is still in memory to handle keyboard interrupts.

You now have a DOS keylogger.

In the good old days memory resident programs were programs, that remained in memory, while other programs were being run.

It was a way around DOS' limitations at that time and not neccessarily a bad thing. Although this method was used by viruses, too.

https://en.wikipedia.org/wiki/Terminate_and_stay_resident_program