These Definitive Editions are part of the newly-announced Mafia Trilogy, a bundle that combines all the franchise's existing games into one concise package. Mafia II's Definitive Edition includes all of the DLC, outfits, and vehicles added to the title over its lifespan, and the same goes for Mafia III's Definitive Edition.
If you already own copies of either title on Steam, you should have already received their respective Definitive Editions for free (playable now), even if you never bothered to snag the optional post-launch DLC. If you don't have the games, you can purchase their Definitive Editions for $30 each. Mafia III hasn't seen any remaster-level visual upgrades, but Mafia II has received "greatly improved visuals and audio."
Mafia I's Definitive Edition is also part of the Trilogy, but it's not playable right now, and there's a good reason for that. Unlike Mafia II and III, Mafia I's Definitive Edition will be a complete remake of the original game. It will have stunning, modern graphics, "enhanced gameplay" (whatever that means), and a $40 price tag.
If you'd prefer not to buy the games individually, you can shell out $60 for the full Trilogy, which is quite the discount. Alternatively, if you're only interested in the updated version of Mafia I, you can pre-order it right here ahead of its August 28, 2020 launch date.