Who is Freewheel? FreeWheel is a video ad tech company focused on connected TV. They were purchased by Comcast in 2014 for $320m.
Why is Disney switching? The official story is that Google is a better fit. The speculation is that this is a direct result of the recent Comcast/Disney battle over Fox.
Why does this matter? Disney is making a huge push in the OTT space and this gives access to major TV players content (ABC, ESPN, etc.) which is what Google has been after all along.