In many stories, an antagonist or villain changes their behavior or attitude for the better and may become one of the good guys. In other stories, they simply stop doing bad things. The reasons can vary, such as it's in their best interest at the moment to stop or what they were trying to do has changed so there's no reason to continue doing bad things.
In both of these situations, many times the characters are said to have reformed. However, there is usually not enough information in the story to support that. Saying they reformed is an opinion that is backed up by usually not anything more than "they aren't doing bad things right now".
Reforming is more than that. It is being shown new information that makes you stop and think about what you're doing, and it leads you to make a conscious decision to change your behavior. It also requires that the story shows your future actions that prove you haven't gone back to your previous behavior.
Many stories do not continue to the point where we would see those future actions that prove someone has reformed. With sequels and spin-offs, the story continues and we can see what happened to that character. But without the further information, the best that can be said is a character probably reformed, based on their behavior and actions at the end of that story.
In the situations where we only have one story (one movie, one book or a single episode of a TV show), care must be taken to avoid labeling a character as reforming if it is really a case of them temporarily stopping doing bad things. If all we see is that they stopped doing bad things but nothing where they were confronted with information that forced them to change, or nothing of their future actions, they did not reform.