The Esc key stands for Escape. It is called escape because it escapes
from the last thing you did. This is not always true, and you may sometimes
need to do CTRL + Z or Edit > Undo.
Esc is from the olden days when you needed to quickly stop a computer
doing something (like deleting all your files) but could not tell it quickly
enough. It is similar to the Break
button. Now it is used to undo selections in Menus.