The alpha value describes how opaque or transparent each pixel should be when composited over another image. Fully opaque pixels hide the background, fully transparent pixels reveal it and intermediate values create soft edges, shadows and glows.
Transparency must be supported by the complete format and export path. A canvas may look transparent inside an editor while a normal MP4 export replaces that transparency with a solid background because its chosen codec and container do not preserve alpha.
ELI5
An alpha channel is an extra layer of information that tells a picture or video how transparent each pixel should be. It lets an overlay show its subject while the content underneath remains visible.
For example, a lower-third title can have solid letters, a soft shadow and completely transparent space around them. If the export loses the alpha channel, that empty space becomes a visible rectangle instead.
