As for the ad, it offers you to upgrade to the full version where you will have lots of useful things such as Pro Tabs, metronome, tuner and many more.
You can read more about in-apps here: http://support.ultimate-guitar.com/knowledgebase/topics/76590-android-apps
You can always close this ad by pressing "back". It only appears three times. (if you're not trying to use functions that are not available in basic version)