Alpha channel is the transparency information in a bitmap file.
PNG with alpha is also called 32-bit PNG. It has 3 8-bit channels for RGB, and one for transparency - the alpha channel.
If you are not experienced with Photoshop or Animate, we strongly advise against trying to design workflows that involve both.
Get very comfortable with Animate's vector tools for creating art and rigs first, and only after that attempt bringing raster stuff in.
https://flash-powertools.com/character-rigging-for-flash-animation/