Okay, so imagine you are playing a game with your friends and you are the boss. You get to decide what rules to make for the game, right? An executive order is kind of like a rule the boss (the President of the United States) makes for people who work for the government.
Executive Order 11478 was made a long time ago, in 1969. It said that the government couldn't be mean to people who wanted to work for them, because of things like their skin color or if they were a boy or a girl. This rule made it so that everyone who works for the government has to be treated the same, and be given the same chances to get a job.
So just like in your game, everyone who works for the government has to follow the same rules and play fair. That's what Executive Order 11478 is all about!