The event is almost certainly to announce a new gadget for streaming video to your TV from services like Netflix, Hulu, and Amazon's own video library. Several outlets have reported recently that Amazon will unveil such a device this spring.
The device is said to run on a modified version of Google's Android operating system, sort of like Amazon's Kindle Fire line of tablets. There could be a gaming element to the gadget too. A few weeks ago, a shot of an Amazon-branded video game controller leaked. It's possible you'll be able to download Android games to Amazon's device and play them on your TV.