Preventative means meant to stop something before it starts — the same idea as preventive. A preventative visit to the doctor, preventative care of a machine, a preventative step against fire. You can also use it as a noun: a medicine or an action that keeps trouble away. Though actually, many teachers and editors prefer the shorter word preventive, and treat preventative as a bit extra. Both are real and widely used; preventative just feels a little heavier, a little more spoken-aloud. Neither word means you already stopped the thing. They only mean you are trying to head it off.