What is the Venus protocol collateral?

Could you please elaborate on the Venus protocol collateral? I understand it's a crucial aspect of the Venus Protocol, but I'm not entirely clear on what it entails. Could you provide a concise explanation of what it represents, how it functions within the system, and why it's essential to the overall operation of the Venus Protocol? Additionally, are there any specific types of assets that can be used as collateral, and what factors determine the eligibility of these assets?