I was just curious as I was reading the possibility of transferring characters during the older MM games before The Mandate of Heaven. Is it possible to transfer characters from MM6 to MM7? Also, how do you mod a game and what exactly is that? I've heard a lot about modifying games and I am interested in learning more.

You can only transfer characters from MM1 to MM2 and from MM4 to MM5; the other games don't support this.

Modding is changing the game in some way. The complexity of modding depends on the game. See Might and Magic mods, projects and stuff for some of the mods related to this series.

Currently there is a lot of helpful tools for modding the games from the MM series. However, there are not much modders left, and some projects which had the teams never saw the light. It is effort-consuming, you know.

Game rules of MM 7 differ from game rules of MM 6. E.g., MM 6 doesn't have races, meanwhile MM 7 have additional layer of skill restrictions (and there were some new skills, while some other were removed), MM 7 featured different system of promotions, which were also tied to choice of Dark/Light Side. Transferring characters wouldn't mean much (and IIRC MM 1 -> M 2 transfer didn't transfer much as well).

The only true character "transfer" in MM series was in MM 4 -> MM 5, but those 2 games had exactly same game mechanics.