what does it mean when it says standard messages and data rates may apply?

[incandescent]

I think that "message" rates means rates for texting, so if you have unlimited texting you should be okay there. And data rates means however much you are charged per megabyte of data transfer (like for downloading videos, or anything else).

I don't know that twitter would add a significant data load, but of course you should check with your own provider if it's a concern. One thing you could do is turn twitter on and then check your own account from your phone and see if you start to accrue extra charges.