If it's what I'm thinking of, it means you sent a text message that wasn't delivered. Open your messages, press 'Menu' and then choose 'Undelivered'. From there, if you do a long-press on the message that wasn't sent, a list of options will come up that includes 'Retry sending'. But you might not need to - for some reason, I sent a text to my friend today and although she got it just fine, it still showed it as undelivered. Weird!