A mod scene is a public moderator announcement.
This can be as sparse as a mere role flip, or the moderator can type long and detailed flavor pieces about the goings-on. However, as a general rule, Less Is More. Moderators who want to embellish the flavor based on players' private actions tend to give away more of the setup than would be good for balance. For instance, revealing that a player was saved by a Doctor guarantees that there is a Doctor in the setup, reveals to the Doctor that they definitely stopped a kill, and ensures that the Doctor's target is probably Town.
In the case of events that occur during the Day phase, it is for the best that the moderator not lock the game thread (thus halting the Day play until the event has been announced) until they know they are going to have to make a mod scene. This is especially true for modkills, where locking the thread is an acknowledgment of a potential problem.
See Night Scene for a more flavor-based description.