SHA256 Inner vs Outer digest?

I know its not really Lede related, but for my project I would like to know the difference or use of inner vs outer digest (hash). Google doesnt seem to be my friend.

Could someone explain or point me to a reference for dummies?

See this Wikipedia article...

Excellent!! Seems I didnt search the right keywords due to lack of understanding the topic. For my use case I need to use “inner” for “simple” SHA256.