When dispelling a spell, there are many sources of dispel bonuses that add to the dispel value, making it easier to succeed in dispelling the spell.
The most common dispel bonus is given by the Wizard itself, and is equal to the Wizard's level divided by half. So a Level 1 and 2 Wizard gains a +1 bonus, whereas a Level 3 and 4 Wizard gains a +2 bonus.
Other dispel bonuses can come from magic items, special rules, units or terrain. Note that for whatever reason, no dispelling attempt may have more than a +5 dispel bonus in total.