According to BBC News:
The royal colonels – the Prince of Wales, colonel of the Welsh Guards, the Princess Royal, colonel of the Blues and Royals, the Duke of Cambridge, colonel of the Irish Guards and the Duke of York, colonel of the Grenadier Guards – all rode on horseback as part of the parade.
Trooping the Colour originated from traditional preparations for battle and has commemorated the birthday of the sovereign for more than 250 years.
Colours, or flags, were carried, or “trooped”, down the rank so that they could be seen and recognised by the soldiers.
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