To beginners, "Alpha channel" sounds like a sci-fi term, like some kind of video 4th dimension. Simply put, it's transparency, something that can be seen through. For example, if you have a graphic that you want export and overlay on top of your video, you need to export it in a format that allows everything but the graphic to be transparent. Otherwise your graphic will be surrounded by black video. This Rocketstock blog post by Michael Maher is a quick look at some of the codecs and image formats that support an alpha channel. These formats can be used when exporting out of Premiere Pro or Adobe Media Encoder.
Creating an overlay or title sequence that you want to make sure keeps the alpha channel on export? Here are some of the best video codecs and image sequences that will make layering clips a breeze. —Michael Maher, Rocketstock blog