Bob -
The machine needs to know where it is before it starts. It keeps it's position in Program Co-ordinates which are displayed when the program co-ordinates led is lit. To this it adds all the various offsets for programs and tools etc, so the tool arrives at the right place at the right time.
On commercial machines, the first job of the day is to "home" the machine - done in Mach by selecting the various home switches. The machine will move to a position that it knows is 0.0.0 in machine co-ordinates and set the DRO's accordingley. It is now "home" . The home position is probably not any good for machining from, and will certainly not bear any relationship to the 0.0.0 position the program is written from - so each program then has an "offset" written into it, which brings the machine to the correct position to start work on that program.
If you do not have home switches fitted and activated, Mach has been written so that your machine co-ordinates will zero at any position.
You should, therefore, move your machine to a suitable position to start your program, and this is normally (but not necesarily) the 0.0.0 position of the program you are about to run.
The safest way to proceed is then to press "machine co-ordinates" so the button is lit. The machine is now displaying "machine co-ordinates" and then press "RefAllHome" This will zero the machine co-ordinates. Then press the "machine co-ordinates" again, the led should go out. The DRO will display "program co-ordinates" This could be 0.0.0 or could be a random set of numbers from your last job.
Zero each DRO using the zeroX, zeroY and zeroZ - the DROshould again display all zero.
You now have the Machine Co-ordinates and the Program Co-ordinates all singing from the same hymn sheet, and you can start your program run.
Why so much fuss - if you do not have both sets of co-ordinates in a fixed relationship to each other (here they are identical - which is the simplest relationship - i.e. the offset is 0.0.0) then any offsets in your program will not necessarily work correcty, and you will be left wondering why.
There is nothing wrong with starting like this, I have home switches on my lathe, but for some "one-off" jobs, it is easier just to get on with it rather that worry about all the offsets using the homing switches. Fitting home switches is something you can tackle later - if you feel then need to do so.
If you look under Config/Fixtures you will see a fixture table. This has from G54 to G58 then another 250 or so from G59P7 upwards, which allows you write a lot of programs, each with it's own start point. If you were in a long run, and closed overnight, when you came in the morning you would zero your machine and press "GO". The machine would pcik up the offset from the fixture table, move to the correct position for that program and get on with it, without further ado. Some porgrams have the same series of instructions, working from different "offsets" if cutting say, patterns out of a large area of material.
These offsets mgiht change during the day, so when Mach shuts down, it saves all the current fixture list, so it is ready for the next day.