What is 12b-1 fee as the primary sales charge?

Excuse me, could you please clarify what a 12b-1 fee is and how it differs from a primary sales charge? I've heard the term before, but I'm not entirely sure of its definition or its significance in the world of finance and cryptocurrency. Is it a type of fee that investors pay when they buy or sell assets, or is it something else entirely? I'd appreciate any insight you could provide on this topic.