Some people still compare Bitcoin (the network) to a payment processor such as Visa. However, they don't understand that Visa is a second layer, built upon currencies. Bitcoin is the first layer; it's the hard cash. Second layer solutions, such as the Lightning Network, make it go millions time faster.

Picture it as following; bitcoin is a type of metal and you ought to store it in a safe place. However, you can't pay with a metal for everyday transactions. Thus, you go to a guy who's willing to give you the option to give him a portion of your bitcoin in exchange for the ability to create agreements with other people who have also given their bitcoin this way. Now you can transact without touching your metal at all. It's faster, cheaper and smarter way to transact.

That being said, add that this guy cannot cheat or lie to you in any way. You've handed out your coins, but they can't steal them from you. You're free to withdraw their promise for BTC anytime you want without their permission. (Which means that, essentially, it's not a promise)