What is a mux in fiber?

Could you please clarify what a mux in fiber refers to? I'm not entirely familiar with this terminology, so I'm curious to learn more. Is it a specific type of hardware or software used in fiber optic communication? How does it function within a fiber optic network? Is it essential for the transmission of data over fiber, or is it an optional component? I'm eager to gain a better understanding of this concept and how it relates to the broader field of cryptocurrency and finance.