CNC milling machines differ from additive machines like 3D printers in that they don't build parts from the ground up, but rather remove excess material from a block of stock to create a final product. This means that the starting point for a CNC milling machine may not always be the build plate, but rather anywhere within the machine's build space.
To ensure that the machine knows where to begin its work, it's important to specify the location of the stock material. This is known as "homing" the CNC mill, which involves activating the limit switches on the X, Y, and Z axes. However, this starting point can be inconvenient to work with, so the work coordinate system (WCS) allows you to define an origin point anywhere you choose. The WCS is usually set using the G54 command, which offsets the machine's coordinate system.