Why is Polkadot a layer-0?

I've come across the term "layer-0" being associated with Polkadot, but I'm still trying to wrap my head around it. Could you explain why Polkadot is considered a layer-0 blockchain? It seems to have some unique features compared to other blockchains, but how does that specifically translate to being a layer-0? I'm particularly interested in understanding how its architecture and functionality contribute to this classification. Thank you for your insights.