The Twelfth House is difficult to understand. It is related to anything which is hidden, such as secrets, secret enemies, or people who betray us. It also has a link to hospitals, prisons, or other places of confinement. On a slightly different tack it is related to self-sacrifice, and to subconscious, repressed and unintegrated energies and emotions.
Some astrologers link it to negative karma.
The planets in the Twelfth House show those energies in our psyche which are suppressed, and which we have not yet integrated into our conscious experience.