So think of your bed an a perfectly flat (no warping) plane that if you tilt it in any direction or combination of directions the G28 reads if one side is higher and/or a corner is higher than any of the rest and so on. That is ABL There is a newer method called Mesh leveling or MBL that you might here around the forum. That is done once and the results are stored in the EEPROM. No need to repeat for each print. This method actually looks at the bed differently. It takes into account if there is any warping found while doing the probing routing. So if your outside edges are all higher than your middle because of a warp for example, it compensates for that. A pretty slick and up and coming standard to bed leveling from what I have been reading from others that are using it today.