Are you thinking a postback? If so a postback is posting back to the same page with a web form to run a method or someother function that is on the page.
Back in the old vbscript days we would post to another page to do something then move to another page. Like a login script, the user would visit a login page and would enter their user name and password. When you clicked the login button you would send then to another page that would process their login, if it failed you send them back if it passed you send them through to another page.
But we now have asp.net so now you have the user login and post back to the same page to process the login, if it passes send them on if not just reload the page. This is just one small reason you will do a postback.
From the wiki: In computer programming, a callback is a reference to executable code, or a piece of executable code, that is passed as an argument to other code. This allows a lower-level software layer to call a subroutine (or function) defined in a higher-level layer.