One of my favourite sites on the internet is Daring Fireball by John Gruber. John is a famous technology writer and UI designer and all-around well-respected internet citizen. He even worked at Bare Bones software, creator of BBEdit, which POPSUGAR was original coded with in 2006. I still code with it today.
Apps in the background are effectively "frozen", severely limiting what they can do in the background and freeing up the RAM they were using. iOS is really, really good at this. It is so good at this that unfreezing a frozen app takes up way less CPU (and energy) than relaunching an app that had been force quit. Not only does force quitting your apps not help, it actually hurts. Your battery life will be worse and it will take much longer to switch apps if you force quit apps in the background.