A navvy is a special kind of worker who does very hard physical labor, like digging, carrying heavy things, and building structures. They were very important in the past when there weren't machines to do this kind of work, so people had to use their muscles and strength to get things done.
Think of it like playing with toys in a sandbox or digging in a garden, but on a much bigger scale. A navvy works on big construction sites and does things like digging tunnels, building dams or bridges, laying down train tracks, and moving big rocks or heavy machinery from one place to another.
Navvies are very strong and skilled workers who need to be in good physical shape, because this kind of work can be very demanding and tiring. They wear protective gear like hard hats and work boots to keep them safe while they work.
Overall, navvies play an important role in building the important infrastructure that we use every day, like roads, railways, and buildings, by using their brains and their brawn!