Adding fields to register.php and form validation

ok, i'm a little new to this so be gentle

i want to add a few fields to the register.php process--things like street address, etc. I know that when fields are left blank you get a javascript window that says: "Username is a required field in this form" when you click the Register button. I assume this is triggered by this:

onsubmit="this.register.disabled=true;if(process_form(this)){return true;}else{this.register.disabled=false;return false;}"

So how can i add my extra fields to that command. More generally, could someone explain this line of code briefly, or link me to another thread about it. I greatly appreciate it.