Holds are sometimes placed on people when they are taken into custody and they are on parole (parole hold) or they are not a U.S. citizen (Immigration hold).
I don't practice in WA. It means that there are warrants from other jurisdictions from the one he is in that have to be cleared up before he can be released.
Edward J. Blum
It typically means that the person is wanted in another county, state, or federal branch of the government. The corrections officers at the county jail can, and sometimes will tell you who put the detainer (hold) on your person. It also means that you'd be throwing your money away on trying to bail the person out of jail since s/he isn't getting out of custody.