What is a layer 1 cryptocurrency?

Excuse me, could you please explain what exactly is meant by the term "layer 1 cryptocurrency"? I've heard it mentioned in the context of blockchain technology and digital assets, but I'm not entirely clear on its specific definition and significance. As someone with an interest in the cryptocurrency space, I'd appreciate if you could elaborate on its fundamental characteristics and how it differs from other types of cryptocurrencies. Thank you in advance for your clarification.