When I browse Erotic Services on Craigslist, occasionally I see an ad that tells you to send a text message (typically one word) to a number, like 45678.  Then it might go onto say something like "get in, then ask for me, jenna4".  

What does that mean?  Is it legit?  An obvious scam?  I don't want my cell phone to be inundated with spam text messages.

You will be charged for a call that leads to no where. Just to be clear, legitimate agencies and independent escorts do not do this sort of thing. Stick with providers who use good old-fashioned phone numbers and email for contact.