What does the symbol stand for on Spain's flag?

The Upper Left: Stands for the old kingdom of Castille. ( gold castle against a red background)

The Upper Right: Stands for the old kingdom of Leon. ( red lion against white background)

The Lower Left: Stands for the old kingdom of Argon. ( yellow and red bars)

The Lower Right: Stands for the old kingdom of Navarre. ( gold chains with red background)

The pomegranate stans for the old Kingdom of Granada, the last stronghold of the moors in Spain.

The Crown on top stands for the royal family.

The pillars are the pillars of hercules which used to mean the end of the world.

PLVS VLTRA on the pillars was added later which is referring to the discoveries of the Americas by Columbus.

The fleur-de-lis is the symbol of the House of Bourbon.