1: A fruit. It can be eaten plain, made into a juice, etc.
2: Rhyming slang, meaning to make a fart sound with the mouth. Usually this is done on the stomach of a child.
Raspberry Tart rhymes with fart. Tart was dropped, leaving the less conspicuous, less gross sounding 'Raspberry'.
Person A: I just love raspberries, but I'm so allergic to them!
Person B: My child would NOT stop crying, so I blew a Raspberry on his stomach. Made him giggle. Works every time.