GRBL and UGS ??? I don't understand

[Neale]

GRBL is motion control software that runs on Arduino. It takes in gcode - think "move cutter to X=10, Y=25" coordinate-type instructions. It turns these into the pulses that the motors need to move the actual machine axes, synchronizing, moving round curves, etc. It's quite a complex task. However, the gcode will usually start on a PC somewhere. This has to be drip-fed into GRBL as GRBL can only hold and deal with a few seconds-worth of gcode and that's where UGS comes in. It is software that runs on a PC and feeds gcode as required to the connected Arduino. It also provides the user interface that lets you select gcode file to run, home the machine (via commands that UGS sends to GRBL), set workpiece zero, and all the other little housekeeping jobs involved in running a CNC machine. No overlap between them - they are complementary and both are needed in a setup like the one you are planning.