Everything in HTML is made with tags. That's how you tell the browser were the things are. Its like putting traffic cones on the road to help drivers to go somewere or to define a specific area.
Each tag allow to do a particular thing or define a particular zone in the page.
So how tags works? It's simple: you write the start and the end of the markup, simply where it needs to be.
For example imagine someone has made an apple pie and asks how big you want your slice. So you use your fingers to tell them where the slice starts and where it ends so the person can cut out the perfect slice for you. It's the exact same thing for web pages, but instead of using your fingers you will use a HTML tag to start the markup, and another one to end the markup zone.