Encryption goes all the way back to Caesar and was designed to make sure that only people with the right key could decrypt and understand the message. But, how does it work? 

Imagine that Nicola has been really excited for her holiday and looking forward to it for months. In fact, she's been so excited that she rushes to get to the airport, but when she gets there, she realizes she's left her luggage at home. This is obviously not ideal, but she speaks to her airline and they tell her that she can have the bag dropped off later that day and they'll have it delivered on the next flight. 

Nicola calls her mom to explain the situation. She tells her mom where she's left the bag and when it needs to be dropped off by. Her mom heads up to her room and discovers that not only did Nicola leave her bag at home, she'd left it wide open on her bed. She closes it up and makes sure to lock it. A few hours later, the bags have been dropped off, safe and on its way to Nicola. 

In transit, the contents of Nicola's luggage are safe because the bag's been locked. When Nicola picks it up easily because she knows the key. Symmetric encryption works a lot like this. A message is encoded in a way that only someone with a key can decrypt it.