Chapters 26.2.3 and 26.2.4 give information on that.
Basically, Absolute mode is used for controlling things where 0 - 127 are always representative of specific points of a control parameter. If we used a Filter Knob as an example, by default, 0 would be the bottom end, and 127 would be the highest point of the filter. Any time you hit 0 or 127, you always get to that specific point, if you hit 64, you would end up in the middle. Primarily this setting is used for knobs that have a limitation to their movement (or sliders). Or buttons that you expect to consistently recall a specific preset (like selecting waveforms on a synth, or types of filters). A big negative quality to this method is that if you were to change layouts on your controller, move your knob around, and then go back to the layout where the knob was controlling say, the filter. The knob would be at a different new position than what it was at when you last used the filter. So when you touch the knob, the filter would abruptly jump to the position of the knob, without permission
You can sometimes get a neat effect by doing this, but it is mostly not preferred haha.
Other types of controls are relative. It is good for changing things UP or DOWN from whatever position a parameter is already at. On a filter, any time you move the knob clockwise, the filter frequency setting would go up from whatever point it is currently at. It is convenient because when you move a knob, your parameter doesn't jump abruptly to the new point, and you do not have to fiddle around with a 'pick-up' mode. This setting is mostly used for supported Endless Rotary Encoders (such as those on the APC). There are several kinds of relative modes, each utilizes different methods of determining movement.