A megalith is a really big and heavy rock that people used to build structures a very, very long time ago, when there were no tools or machines to help move them. Imagine you want to build a big house, but instead of using tools like hammers, saws, and cranes, you have to move a really huge rock that is heavier than a bunch of elephants!
People who built these structures, called megaliths, were very clever and strong, and they knew how to move these big rocks by rolling them on logs or by dragging them with ropes and lots of people. Megaliths were used to build all sorts of things, like big circles for ceremonies, tombs for important people, or walls to protect their towns.
Even though they were made a very long time ago, some megaliths can still be found today, like the famous Stonehenge in England!