CLI – As mentioned before, every command line command has a descriptive man page entry that you can access. That is often a very long and detailed description of the command and all of the possible options available to it.
Sometimes you just want a quick explanation as to what a command does. This is where the whatis command is useful. It will return a one line description of the command being referenced.
The description of whatis in the macOS man page is:
whatis searches a set of database files containing short descriptions of system commands for keywords and displays the result on the standard output. Only complete word matches are displayed.
To learn a lot more about how to use whatis, take a look at “Linux whatis Command Tutorial for Beginners (5 Examples)“. Yes, this is a Linux article, but the whatis command as implemented on macOS is fundamentally the same. Check the macOS man page for whatis for all of the available options.