It's caused by the page you're trying to reload.
For example, when you post to this forum, you type your message into the box at teh bottom and click the Post button. When you click that button, your browser sends the contents of the text box to the mac-forums web server, which then displays the page with your new message.
If you then click the Reload button, your browser will repeat the last thing it did...in this case, sending the contents of that text boxagain. Most of the time, you really don't want to send two copies of the same thing, so Safari will ask you.
This only happens on pages which are created based on some form you submit (as opposed to typing in an address or clicking on an ordinary link.)