What does Fleet mean in regards to purchasing a vehicle, other than it was used by corporate or rental company. Does it decrease the value at all?

Have you seen the prices lower? I wouldn't buy a vehicle that was a rental. If the price was lower, it wouldn't be by a significant amount. Maybe a couple hundred. Usually fleet vehicles are maintained mechanically but driven by many people. If you are considering one, have it checked out by your mechanic first before purchase.