The Moon is your inner weather: the part of you that reacts before you think, and what you need to feel safe. The 12th house is where you go to be alone and what you keep there. When the Moon sits in the 12th, your emotional life is largely a private one. You need to disappear sometimes, not socially but fully, and that need is not a flaw in how you are built. It is the thing that keeps the rest of it running.
The 12th house is cadent, one of the four houses that come before the angles, where things are learnt and adjusted rather than announced. Planets here tend to work below the surface. With the Moon here, your feelings often process in private before they are ready for anyone else to see. People who know you well will still sometimes be surprised by what you were going through.
The 12th house is Pisces's natural house, linked with Neptune, its ruler. Neptune softens edges and dissolves boundaries. That sits well with the Moon in some ways: you pick up on other people's moods readily, sometimes before they have named those moods themselves. It can also make it genuinely hard to know where your feelings end and someone else's begin.