What is a RTN fee?

Can you explain what a RTN fee is, in simple terms? I've heard it mentioned in relation to cryptocurrency transactions but I'm not entirely clear on the concept. Is it a fee charged by a specific entity, or is it a standard fee that applies across the board? And if it's charged, what exactly does it cover? I'm trying to get a better understanding of the costs associated with cryptocurrency transactions and how they differ from traditional financial transactions.