Cantrips are at-will spells for you.
Unlike normal wizards, who use a standard action to cast a cantrip, you can cast a cantrip as a quick action.
To do something particularly cunning or surprising with one of your cantrips where the GM isn't sure whether you could pull off that use of the spell, roll a normal save (11+) to cast the spell the way you envision it.
Additionally, you can expend a 3rd level spell slot or higher to choose one cantrip per spell slot you have given up and create a once-per-day related effect with it that is much greater, if you and your GM can agree on a cool effect that suits the cantrip.
You can use cantrip-style versions of any wizard spell you have memorized. When you expend a spell, however, you can't make cantrip-style use of it any more. The key is that none of these uses should be combat relevant or deal damage.
The Cantrip Mastery talent is more about enhance the roleplaying and less about combat usefulness.