What is a 12b-1 fee disclosure?

Could you please explain what a 12b-1 fee disclosure is? I understand it's related to mutual funds, but I'm not entirely clear on how it works or why it's important for investors to be aware of it. Is this fee charged directly to investors, or is it taken out of the fund's assets? And how does the fee impact the fund's performance over time? I'd appreciate it if you could provide a concise yet comprehensive overview of the concept.