What is an excess TX fee?

Excuse me, could you please clarify what an "excess TX fee" refers to in the context of cryptocurrency transactions? I'm not entirely familiar with this term and would appreciate a concise explanation. Is it a fee that's charged when a transaction exceeds a certain size or complexity, or is there another reason for its existence? Additionally, how does this fee impact the overall cost and efficiency of a cryptocurrency transaction? Thank you for your time and assistance.