Is Polkadot layer zero?

I've been hearing a lot about Polkadot lately, and I've seen some references calling it 'layer zero'. Could you clarify what that means? I understand that blockchain technology often involves layers, but I'm not quite sure how Polkadot fits into that picture. Is it really a layer zero solution, and if so, what does that signify in terms of its functionality and position within the cryptocurrency and finance landscape? Could you please provide a concise description of Polkadot's role as layer zero, and perhaps also explain how it differs from other blockchain platforms?