As the title says this is a network adapter for the Playstation 2. This allows you to connect the fat PS2 to the internet but also lets you use a Hard Drive in your PS2. Using this and a softmodded memory card you can use HDLoader on your PS2. That will allow you to back up your games onto the Hard Drive which saves on laser wear and gives you quicker loading times for your games.
No special modification of your console is required as this fits in the expansion bay, and yes this is completely legal
Will accept paypal, cash on collection (from Gravesend) or BT (HSBC or Natwest)