What is an alt layer?

As a cryptocurrency enthusiast, I'm often fascinated by the intricate layers and dimensions within the digital currency landscape. Could you elaborate on the concept of an 'alt layer'? Is it a specific technological advancement or a conceptual framework? How does it differ from the main blockchain layer? Do alt layers provide alternative functionalities or services? Furthermore, are there any specific alt layers that have gained popularity or recognition in the crypto community? I'm keen to understand its significance in terms of scalability, interoperability, and the overall cryptocurrency ecosystem.