What is the meaning of the Union Jack flag?

The Union jack flag is made up from the flags of the United Kingdom.

It should be noted that the correct term is Union flag. It does not become the Union Jack until it is positioned in the corner of the flag, as in the Australian and New Zealand flags. It is often stated that a flag consisting of just the Union Jack may be given this term when flown from a ship's mast at sea. Controversy remains over this particular assertion. (see the related link)

Interestingly, the "Jack" flown by ships in the US Navy is also known as the "Union Jack".

Either way, there is no such flag as the "Union Jack flag".