It depends what the item does. Generally it potentially has the following effects

+1 = ac+1, wc+1, weight -10%, dam+0.1 (weapon only)

Note that this only applies if it already has that stat on the item. So an item with only ac will only give ac bonus and not wc/dam bonus. Higher +'s give a multiple effect. An item with ac0/wc0/dam0 will give no bonus other than being slightly lighter and having a slightly higher selling price

It should also be noted that items randomly generated only go up to +3. Special drops can go higher but in that case weight reduction doesn't apply