Exactly what it says, which is providing a platform for companies to share videos for a variety of purposes. This was a topic of conversation at one of the companies I worked for, which is a large payroll service provider. The company has a series of offices distributed in many locations around the nation, and whenever a new training or motivational video was release, there was always a question of where to host it. Hosting it in the company's datacenter wouldn't work, as that would consume a lot of bandwidth, and building out a local CDN can be expensive if you don't want to rely on someone else.

Stream is basically providing an internal Youtube, a place where you can upload videos, control who access to them, and let Stream take care of routing, etc.