Let’s imagine a USB thumb drive. You’ve probably used one, and you can just plug it into your computer and copy files onto it. The great thing about a usb thumb drive is that you can carry it with you and you can view your files on a computer anywhere as long as you have the thumb drive with you.
Now imagine all the files that make up your website are on that thumb drive and instead of carrying it around with you, you give it to a company who plug it into one of their powerful computers that is connected to the internet. Now you can view your website files from anywhere as long as you’re connected to the internet.