What exactly is standard messaging rates?

[Life Is Beautiful at Sixx: A.M.]

I re-activated my twitter yesterday. I selected a few people I'm following to receive their "tweets" via texts.
Twitter says they don't charge for this service, but standard messaging rates apply. I have unlimited texting. And my carrier is AT&T. When it says standard messaging rates apply what exactly does that mean? Does AT&T charge you separately for it, or does it just go into my unlimited texts

[sbeck14]

You're covered. Standard messaging rates apply basically means make sure you've got a messaging plan, because you're going to be charged for a text message just like you would be if a friend sent you one.

[Stu]

'standard rate" are the charges you either get or pay for any text message...they are usually $.25 / message or something like that unless, like you, you have unlimited texts...you shouldn't see any increase in your bill because of these messages.