A Hotkey is simply a keyboard shortcuts to a command. Hotkeys are an important aspect of the game as they can often be quicker than using a mouse. When competing at a higher level every second counts, so the less time it takes to execute a command the more effective you'll at attacking, defending and building. Learning hotkeys will speed up mastery of the game so it is wise to invest time in learning them. You will occasionally however find a few top players who work without all of them. It is quite common for players to customise their own hotkeys rather than using the default keys for efficiency. Ideally, one hand should be on the keyboard and the other on the mouse, and it is more convenient for hotkeys to be within the left hand's reach without having to stretch too far. Hotkeys Cheatsheet

For a more indepth guide see Winchester's hotkeys video