When Mach ( the controller ) is started it does not know where Machine coordinate 0,0,0 is so the user must define it to the controller.
Normally 0,0,0 would have the table positioned to the left and nearest the operator with Z raised. You would define that position as HOME / Referenced.
Home can be any where but the above is conventional. The controller now knows where the machines 0,0,0 is and any move away from that position would be positive X & Y . Soft limits restrict the axis movements.
When Mach is started the default Work / Fixture offset G54 is used, thus if there is no Work /Fixture offset then, Machine coordinate 0,0,0 = Part Coordinate 0,0,0.
So have read of the Manual or again watch the video, with the above explanation in mind, and know what each input for an axis in Homing & Soft limits defines to the controller.
RICH