Millwork is basically like building with LEGOs, but instead of plastic blocks, it's made of wood or other materials. When you're building with LEGOs, you can connect the pieces together in different ways to make buildings, vehicles, or whatever you want! Millwork is like that, but instead of connecting plastic blocks together, carpenters use special tools to cut, shape, and connect pieces of wood to create things like furniture, doors, windows, and cabinets.
The reason it's called "millwork" is because once upon a time, the wood used for this kind of building was milled - which means that it was cut and shaped using special machines in a factory called a mill. Today, a lot of millwork is done by hand or with newer, more advanced machines.
When you go to a fancy hotel lobby or someone's really nice home, you might notice lots of fancy millwork around. For example, the baseboards along the bottom of the walls might have fancy designs carved into them. Or, the doors might have intricate patterns cut into them! This is a form of millwork that adds a lot of character to a building.
So basically, millwork is the process of cutting, shaping, and connecting pieces of wood to create beautiful and functional things for buildings.