Best explanation of shields in Valheim especially after the H&H update last year is in a video - "How Shields Work In Valheim After Hearth And Home - YouTube · FireSpark81 · 18 Sept 2021" - not sure if I can post direct link, but trying below:
https://www.youtube.com/watch?v=uw33dzhz21c

As I understand it, Valheim's combat is more or less in the Dark Souls lineage style - should be some players familiar with that who will hopefully chime in.