Can you please explain what this code does?

If I run:

DO ##class(%SYS.Namespace).ListAll(.result)
zwrite result

I get:
result("%SYS")=""
result("USER")=""

....

So how does this code work?

if $d(^|"%SYS"|CONFIG("Namespaces"," "))

for set ns=$o(@$ZR) quit:ns="" write ns,!

In reference to your ObjectScript code when I try to get the databases contained  in namespace "USER":
SET status = ##CLASS(%SYS.Namespace).Databases("USER",.dbInfo)
^
<METHOD DOES NOT EXIST> *Databases,%SYS.Namespace

Please advise

Thanks to everyone for the fast reply!