An HTML tag works like a kind of switch. You switch something on or off in your browser. Tags can be written in uppercase as well as lower case. It is probably wisest to always use uppercase, to make them stand out in your code. But in this you're free to do as you like, both work the same in your browser.
Tags often come in pairs.
The opening tag sets the switch to on, the closing tag turns it off again. Sometimes tags have one or more attributes . These supply extra information to your browser. Sometimes you can add data to an attribute. When putting a picture on a page, you would use an image tag.