What does it mean when your TXN is declined?

I'm curious, can you explain to me in simple terms what exactly it means when a TXN, or transaction, is declined in the world of cryptocurrency and finance? I've heard about it happening but I'm not entirely sure of the reasons behind it or the potential implications it may have for users. Can you elaborate on the possible scenarios and factors that could lead to a transaction being declined, and how users can go about addressing the issue if it occurs?