You should think of browser window is a blank page or sheet. What you can do with a sheet or blank paper. You could draw the picture, write small text, big text, paragraphs and etc...
the same we are going to do with the browser page. In order to write text, paragraphs and draw the picture, HTML help us to do it easily.
Lets try to write a big text in a empty browser page(kind of header text)
create a new file called "headerexample.html" and type the below lines.
<h1>Welcome to my page</h1>save the file and open it in browser, you see "Welcome to my page" in big text.
What is <H1>?
Whatever we start with < character and end with > character called tag. Here H1 represents header tag level 1.
How many levels of header tags available?
There are 6 levels which is called <h1>,<h2>, <h3>, <h4>, <h5>, <h6>