What does "off you go" mean?

In common usage, the phrase off you go has a patronising or dismissive connotation.

It is something you say to dismiss a child - Off you go to school now or Off you go and tidy your room.

It is not used in addressing a superior, a customer or in similar situations where you would not wish to be thought of as even slightly rude.

The meaning of “off you go” is somewhat difficult for an English language learner for two reasons. First, off has many meanings; second, the word order is unusual.

Off in the context of this phrase means away, with a sense of definite separation. You will also hear this sense of off in phrases about leaving or creating distance, such as “walked off”, “drove off”, “chased off”, and “off with his head”.

The more conventional word order for the command would be “you go off” (or “go off”), meaning “go away”, but the word order “off you go” is more idiomatic.

See:
New English Dictionary, volume O