What is debug?

Debugging is a generalized term which essentially means to step through a process in order to systematically eliminate errors. It takes on several different meanings depending on what you are debugging:

In programming, debugging is done when undesirable program execution or termination occurs. IE you may be coding a financial package for a bank and find that your depreciation portion of your package is computing incorrectly. You would then debug your code to find your math error that is causing the incorrect computation.

In IPL (initial program load) of an operating system, you would debug certain drivers, start-up programs, etc to figure out why the OS does not boot properly.

In hardware (a bit of a stretch, but still) you might debug why a computer does not load at all by replacing memory, then trying the boot sequence, seeing if there is a change, remove network card, try boot sequence, no change, etc until the machine boots properly.

The term originates in the early days of computers when computers were large monstrosities constructed of large vacuum tubes. It was common for actual physical insects (bugs) to cause problems with the machines, hence you would "debug" (literally!) the computer.

If you mean the Debug command, it was a program included with MS-DOS. It would allow you to create small programs using machine code, execute ROM addresses on controller cards, and patch programs. It was an advanced programming tool and not intended for most end users.

In programming, debugging refers to troubleshooting software to discover why undesirable operations or mistakes are taking place or why the program crashes.

The term debugging comes from the early days of computing where lots of switches and vacuum tubes were used, and insects got into the switches. So literal bugs caused problems, and when the bugs were removed, the programs worked fine. Now, the term bug is more of a metaphorical concept, where the problem is not insects, but unintentional mistakes made by programmers.