Netflix currently uses Microsoft's Silverlight plugin to run its streams. It's an inelegant solution that sometimes makes for subpar and bogged down streaming experiences. But Silverlight is possibly dying! And Netflix wants to get better! So Netflix is planning to move on from streaming on Silverlight and towards HTML5 video. When that happens, streaming movies on Netflix might suck less.
Netflix detailed its goals to move away from Silverlight because of the limitations of browser plugins in general: Users have to install a plugin, users may be wary of security and privacy risks that come with plugins, and probably most importantly for Netflix, not many mobile browsers support plugins. The future for browser plugins such as Silverlight is not bright. Enter HTML5.