What is the meaning mill?

Mill has many meanings:

1. a factory for certain kinds of manufacture, as paper, steel, or textiles.
2. a building equipped with machinery for grinding grain into flour and other cereal products.
3. a machine for grinding, crushing, or pulverizing any solid substance: a coffee mill.
4. any of various machines that modify the shape or size of a workpiece by rotating tools or the work: rolling mill.
5. any of various other apparatuses for shaping materials or performing other mechanical operations.

More commonly used in conversation would be the a Mill used in manufacturing or in food processing.
Many people use CNC Mills, some fit on desks. This kind of mill usually has three or more plains of movement, each plain is known as an Axis.
CNC stands for Computer Numerical Control, each axis is controlled by the computer with the help of Stepper motors and Integrated Circuits on Printed Circuit Boards that connect to a PLC (Programmable Logic Controller) or Computer.