Need to give out my UDID. Where do I find it on the iPad?

I downloaded a free text message app called textnow. Since I had my iPad replaced with a new one, textnow is requesting that I give them my UDID number so that they can delete my account off the old iPad so that the new user will not be able to access my text messages.

Question is,

+ what is a UDID number

+ where do I find it

+ why does the textnow app developer need it? Can't they just delete it using my username?

+ what can they do with the UDID number