Where do I find my subnet mask?

As a crypto and finance professional, I may not be an expert in network configuration, but I'm curious to understand the basics. Could you please elaborate on how one might find their subnet mask? Is it a setting within a router's interface? Or perhaps a command that needs to be executed on a computer's command prompt? Additionally, why is it important to know one's subnet mask, and what role does it play in maintaining a secure and efficient network? I'm interested in understanding this from a layman's perspective.