Pre-load sets the amount of force required before the shock actually moved when any given a force is applied to it.
Example, you weigh next to nothing and have full (maximum) pre-load on the shock, then the amount of force needed to get the shock moving will be more than the force exerted by your weight down – so the spring in the shock will only move a little bit. This will make the bike feel like the rear end has no suspension at all, kind of like locking it out.
Conversely, if you are a fat bloke and have no preload on the rear, the suspension will be too soft and you will fly through the entire length of the suspension travel on the slightest of bumps, leading to a very soft “wallowing†back end.
So when you load the bike up, you will make the bike heaver so you need more pre-load (assuming you had it set up correctly in the first place). All this will depend on your weight and how much kit you are taking, so a good starting point is to wind on the pre-load all the way up to max (hard) and then back it off a full 1 to 1½ turns. If the rear end feels too hard you can back it off in steps until you are happy. Conversely if its too soft, wind it up a bit.
When the bike is fully loaded it is better to have slightly too much pre-load than too little but don’t get too stressed about it.
