What does the red white and blue symbolize in the british flag?

The Union of the individual countries: England, Scotland, Wales and Northern Ireland is known as the United Kingdom

The Union Jack flag is described as a blue field with the red cross of Saint George (St George is the patron saint of England) edged in white

It also consists of the diagonal red cross of Saint Patrick (St Patrick is the patron saint of Ireland)

The flag is then superimposed on the diagonal white cross of Saint Andrew (St Andrew is the patron saint of Scotland)

The correct name for the flag is the the Union Flag, but it is commonly referred to as the Union Jack

The Union Jack Flag, particularly the Blue Ensign is the basis for a number of other flags including many Commonwealth countries and their constituent states or provinces as well as British overseas territories