What is Rest API?

I have read the documentation from paypal and woocommerce, but I only need to know if it should be enabled in woocommerce or not. Is it strictly for testing paypal with your site?

I do not understand any of the development jargon or concepts ...so I was unable to determine exactly what it is or whether I need it.

I already have an API from paypal.

Does anyone else have a woocommerce enabled site? I just need some brief advice if possible.