What does mega mean online?

Could you please clarify what the term "mega" means when used online? I've seen it in various contexts, but I'm not entirely sure of its precise definition. Is it a reference to something specific, like a large number or a particular unit of measurement? Or does it have a more general meaning that applies to various situations? I'd appreciate it if you could provide a concise and clear explanation of what "mega" signifies in online communication.