When creating a new dynamic album:
Someonw has to decide - at search criteria - between dynamic or tagged.
What does this mean?

Tagged means based on tags, dynamic based on a search criteria. It's also written next to the input field below: "Select tagged to tag the current search results with this tag and use as the album criteria."

Thanks.
I didn't get it for whatever reason.

So if tagged is selected it won't be an "normal" search.
But if "dynamic" is chosen, tags are also part of the "search result".

Yes, if you included tags as a search fields to be covered. Tags can be useful (and less resource hungry even if there is a search result cache if enabled) if you know that specific tags are used.