Every computer on the internet is assigned a unique number called an IP address, and it works kind of like a street address in telling other computers where it's located.
When you go to a website, you're actually accessing the website saved on some company's computer somewhere.
You type a domain name, like "google.com", into your browser to get to that site. DNS is the system that lets your computer figure out which IP address goes with "google.com" so your computer can talk to the computer that runs that website. DNS is like the yellow pages for IP addresses, you look up a name and it gives you back their address.