Mercia was a kingdom in England a long, long time ago, way before you were born. It was ruled by a king or queen who was in charge of the people who lived there. Mercia was in the middle of England, so it was very important because lots of other kingdoms were around it.
People who lived in Mercia were called Mercians and they spoke a language called Old English. They had lots of wars with other kingdoms, like the Vikings and the Saxons. They were very brave and fought really hard to protect their land and their people.
Mercia had lots of towns and cities, like Birmingham and Leicester, where people went to trade and buy things they needed. They had a big army too, and lots of soldiers who protected the kingdom.
Today, Mercia isn't a kingdom anymore, but some people still talk about it and remember the important things it did a long time ago.