Simple refers to something that's easy and uncomplicated, without too many steps to follow.
Simple comes from the Latin word for single (simplus). Simple things are often solo, like a simple dress or directions with one step. A teacher might advise you to KISS (Keep It Simple, Stupid). What could be bad about a word for something easy? Well, it's not that simple. It can be an insult. If you call your little brother simple when he can't tie his shoes, you mean he isn't very smart. Rude!