When someone asks you a phone number, you look it up in the phone book. If someone else asks for the same number, you don't bother looking it up, because you already have it in your head.
Computers do the same thing. There's a cache on your computer's browser, so that if you ask to look at the same page again, it doesn't go and get it again, it shows you what it just showed you.
Usually, that's fine. But sometimes, it isn't. For example, in my chatroom, the page that displays the chat has the same URL all the time, but the content changes. Likewise, the page for the Message Boards. If you need to see what's changed, you can hit your browser's "Reload" button.
To make this work properly, here's what you do.
Older versions
Options ... network preferences ... Cache. Then you can tell it to clear the memory and disk cache now, and you can check the box next to "Verify Documents Every Time"
Newer versions
Go to Edit ... preferences ... advanced ... cache and tell it "Document in cache is compared to document on network every time"
Older versions
View ... Options (or Internet Options) ... Advanced ... Settings ... and then click "Every visit to the page" Then you can tell it to clear the disk cache by clicking "Empty folder"
Newer versions
Go to Tools ... Internet Options ... General tab ... Temporary internet files settings ... Check for newer versions of stored pages every visit to the page. (Or maybe "Automatically").