Modmail is like direct messages, but with a message box shared by all moderators of the same community. Any mod of that comm can see the messages sent to that box, or use it to send messages to the users.

This has a few benefits:

• Typically, users don’t know which mod they should contact for clarifications, ongoing issues, etc. Because they don’t know who’s active, or even who can solve that issue.
• Sometimes a mod needs to issue a warning, but that would be insensitive or impolite to do through comments; for example if it involves the privacy of a third person. Doing so through DMs sounds like the specific mod picking on the user, instead of issuing an official warning.
• It reduces the likelihood of miscommunication between users and mods. For example: user contacts mod A, mod A allows the user to post something, user posts it, mod B sees the post, and remove it. With a shared message box, mod B would see that mod A allowed the user to post it, and leave the post alone.

It isn’t currently a big pressing matter, as current mod teams are kind of small. However I think that it’s necessary for Lemmy’s growth.