Yes the description tells it all.
You can use XNA Libraries in your Silverlight applications.
How?
Well try to create a Silverlight Application for WP7,
when click Project->Add Reference you will see:
Microsoft.XNA.Framework
Microsoft.XNA.Framework.Game
Microsoft.XNA.Framework.GamerServices
Microsoft.XNA.Framework.Graphics
Microsoft.XNA.Framework.Input.Touch
Yes they are XNA Framework Libraries.
Its good to see Silverlight and XNA shares their libraries to work together!