I believe what your talking about are "media markers". In encoder you have the ability to manually define markers in your media. These markers are points in time of your video. In the right panel of encoder there is a section called "media markers" (not 100% sure what its called)

Optionally you can import these media markers from a file, it's just a xml based list of elements of a specific format. Search for "encoder media markers" and you should find out everything you need to know about how to create them

These markers contain a time and a data field. That data field can contain text like in a subtitles type of scenario

In silveright when you play a media file that has these media markers there is an event on the mediaelement, the thing that is playing your media, where you can listen for these media markers. Basically when a marker is reached this event is fired and you can do something like show the data associated to that marker..

Just do a "Silverlight Media Marker" search to get more info and there are great samples out there!

Good luck