The <form> tag is to place the HTML controls, it is also used to send form data's to the server.
This tag is commonly used to retrieve data's given by the users and sends it to server.
It is commonly used in Sign in, Sign up process.
The above html code will call the following php file, when you press the submit button, the values are passed to the below php code, and you can see the php code returned result.
If you dont know php, dont worry. We will see it in php tutorials.