Things done retroactively take effect starting on a date in the past. If your boss pays you retroactively, she'll cover hours you worked in the past as well as the ones you're working today.
If your water bill lists higher rates, and also announces that the new fees apply retroactively to January first, that means you already owe the water company extra money for past months. And if a musician becomes popular retroactively, it means that albums released years ago are just starting to be listened to in great numbers. Retroactively comes from the French rétroactif, "casting back," from the Latin roots retro-, "back," and agere, "to set in motion."