Some browsers use isolation features for plug-ins like Silverlight, and run them in a separate operating system process instead of the main browser process. Visual Studio has problems attaching to these additional processes to provide a comfortable debugging experience for Silverlight projects (and requires you to manually attach to the correct process through the debug menu). In some other cases, Visual Studio also may fail to attach the Silverlight debug engine even if the plug-in is not running in a separate process (I've experienced this when the browser needs to be freshly started and that takes too long).
The Silverlight Debug Helper installs as a Visual Studio Extension and sits in the background until you start debugging your projects. It then checks whether Visual Studio managed to attach to the correct (browser) process(es) to debug your Silverlight projects, and tries to do that if it hasn't. For more information and a detailed description, visit the project page at: http://sldebughelper.codeplex.com/