Please Help!! -- Userlibs

I keep seeing stuff about Userlibs! and i dont understand them at all! can someone please help me out?

userlib template:

.lib "my_DLL_library.dll"
my_Blitz_FunctionName% (param%) : "my_DLL_FunctionName" 

.lib " "
my_Blitz_CustomFunctionName% (param%)  

create a new file : blitz3D/userlibs/myNewFunction.decls

.lib "user32.dll"
GetSystemValue% (index%) : "GetSystemMetrics"

.lib " "
DummyBlitzFunction$ () 

type this code in your BlitzIDE :

Function DummyBlitzFunction$ ()
  return "It works !" 
EndFunction

print GetSystemValue(0)
print GetSystemValue(1)
print DummyFunction$()