A sanitarium is a mental institution that an inmate can leave at his own free will. The song is written from the perspective of one of these inmates - he's afraid to leave because of what's out there.
This is somewhat inspired by the 1975 movie One Flew Over the Cuckoo's Nest, starring Jack Nicholson as a mental patient.
Metallica members James Hetfield, Lars Ulrich, and Kirk Hammett wrote this.
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