Essentially it means low barrier to entry - a person who has been chronically houseless, who may have ongoing substance abuse or mental illness concerns, gets to access housing before they have to be completely sober or fully in treatment and stable (like holding down a full time job, or having completed all of the paperwork to obtain disability).
Give them a roof over their head, in the form of permanent supportive housing, and the costs to the system overall go down — hospitals, jails, etc. It can be controversial because of society’s view that people should have to “earn” certain things, but it’s effective. There’s now decades of published research on it. There’s just not enough political will to support enough of it.