The machine needs to know where it is, and keeps track of itself in memory. When you "set all home" the machine goes back to a fixed position, and sets all DRO's to zero. If you jog too and fro, or up and dpwn, the DRO's will track where you are. This is absolute positioning - and is the MACHINES particular position. If you tell it to go to e.g. X1,Y1,Z0 it will always go to the same position.
That position is not always the best position for machining, or writing code from. For example, my lathe home is tight under the chuck, with the cross-slide as far towards me as possible. This position is no good for starting to write code from, the best place to write code for a lathe is X0 on the centre line of the lathe, and Z0 at the end of the work piece.
To get to that position from "ref all home" you select an increment e.g. g54 to g59. say G56. This adds the requisite amounts to the DRO's from the "ref all home" position, so that the DRO's show X0 Y0 Z0 in relation to your offset position, ready for work. This also applies to offsets it adds for tool length and diameter. This is the incremental position, or offset position and is the one most used in programs.
IF HOWEVER you want to make a move that really has nothing to do with the program, but you need to go to a specific position for the MACHINE e,g, for a tool change, you must revert to absolute co-ordinates to tell the machine (G53) - make the move - then change back to your offset position before resuming.
There are tutorial videos in the collection.