It's a cheery British phrase meaning the equivalent of 'goodbye'. It actually derives from World War One, when English soldiers heard the French saying "à toute à l'heure" which meant 'see you soon'. The average British Tommy had some trouble with that, and so it became 'toodle-oo', and went back into English usage.
Imagine the war in France; the violence, the slaughter, the uncertainty of ever seeing one's companions again. Under those circumstances this cheery mangling of the French phrase carries untold bravado, fatalism, and downright courage. Cherish what the language holds.