PS3 Media servers allow you to connect your PS3 to other media Devices. Which I am asuming means, computers, Steros, Surround Sounds Systems, Home Entertanment Systems and the likes.
Here is a link from the Official website which explains about them.
http://uk.playstation.com/help-support/ps3/guides/detail/item68234/Connecting-to-other-devices/
