When casting a spell, there are many sources of casting bonuses that add to the casting value, making it easier to succeed in casting the spell, as well as making it harder to dispel for your opponent.
The most common casting 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 casting bonuses can come from magic items, special rules, units or terrain. Note that for whatever reason, no casting attempt may have more than a +5 casting bonus in total.