The Managed Extensibility Framework makes it easier to create and reuse extensibility applications by providing a standardized model for discovering and composing application plugins. This allows you to get an off the shelf component model without having to build the infrastructure yourself as part of your application. MEF is going to be available as part of Silverlight 4 as well as downloads for previous releases.
Mike Taulty has recently put up a six part series of screencasts covering the use of MEF in Silverlight 4. Each part leads to the next (after a few minutes of introduction) so you'll probably want to watch these in order or at least have skimmed the previous parts before moving on to the next.