I know it's the combined crosses of St. George and St. Andrew, although I'm clueless as to what the two crosses originally represented, and when and why they were combined.
It also includes the cross of Saint Patrick (diagonal red saltire on a field of white).
That's why the Welsh are so crossed about the flag, they need to have the dragon on it, or a representation of St Davies, then it'll truthly be the "Union" Flag. (although, technically, Wales was annexed by England, so when England and Scotland formed the union, Wales was part of England.)
of course, the crosses were combined/added when England and Scotland first became a union after Liz I, and then the St Patrick cross was added when Ireland became part of the union (1800).
And
Sci, I know it is accepted to call it either Union Jack or Union Flag, that's why I specifically mentioned the Royal Navy.