Autofocus is an boolean attribute of form field that make browser set focus on it when a page is loaded. If you still don't understand what that is, please go to Google and you notice you just type your search string without first click on the big textbox.
This is because Google has set focus to that textbox automatically (I am not saying Google uses HTML5 "autofocus") and save you ONE mouse click.
Here is the code to do it, piece of cake.
<label for="name">Your name goes here :</label><input id ="name" type="text" autofocus/>
"autofocus" is a boolean attribute, it is needless to set any value to it.
As usual, not every web browser supports "autofocus"
Below is actually a demo as well as a test. If your web browser support Autofocus, you will notice the cursor is blinking in the first textbox, if your web browser does not support it, it will be set focus at the second input textbox.