MOTD - message of the day, it's the small text blurb that's printed when you start a new shell instance on linux or mac computers. Not sure if it's a thing on windows.
Which (readline, that is) is used in tons of command line programs, shells, REPLs, etc. to parse user input. So if you know the shortcuts you can apply them in tons of programs...
If you don't use MOTD, putting the shortcuts you wanna memorize in a macOS sticky note works great. Or editing them into your desktop image. Or a legit paper sticky note / label printer label stuck to your monitor - no screen real estate gets consumed that way
Readline refers to the GNU readline library [1]
1 - https://en.wikipedia.org/wiki/GNU_Readline