What is pre keyed?

PowerPoint

Pre-Keyed relates to footage or video effects that have been rendered out with the RGB+Alpha Channels. So it's essentially "Green Screened" Your footage will have no background and can be placed over top of other footage or images, or colours as a separate layer with the "keyed" footage showing as well as your footage underneath it.

Most programs can do this - After Effects, iMovie, Camtasia Studio, PowerPoint, FinalCut Studio, And I'm sure there are others as well.

It becoming more and more popular and software developers are recognizing this. The term "Pre-Keyed" is on a "Breakout" in google's search Trends.