The definition of each word explains the process; structures will be maintained, but only to the extent that they will not be allowed to fall over, or otherwise deteriorate in a major way.
Any building that was standing in 1962, when Bodie became a State Park, may be rebuilt or preserved as the photographs of 1962 showed them. By putting new roofs on the buildings, rebuilding foundations, and resealing glass that is in window frames, the State is able to keep buildings from naturally decaying - thus the term "arrested decay".