Most CNC machines have no inherent sense of their absolute position and where their cutting heads are positioned. Zeroing a CNC Machine is the process of making sure that your CNC is precisely setup for the material that it is going to cut(giving it absolute positioning). For a milling machine that includes making sure that X and Y axis are positioned correctly and that the Z-Axis is calibrated to start cutting at the correct height.
There are multiple ways to do this. Beatty Robotics has written a very nice post about how to use a USB Microscope to align the X / Y axes with a micro dot/hole and using probing to set the Z axis height.
The article shows how easy it can be to set up a machine by using fixed measurements like the location of the USB camera and then using the offset to calculate the exact Zeroing point.
woo great CNC
nice. here is my 3rd homemade cnc. http://myhomemadecnc.blogspot.com/