What is a BLS card?

Could you elaborate on the concept of a BLS card? I've heard it mentioned in the cryptocurrency and finance circles but am not entirely clear on its function and purpose. Is it a physical card that holds digital assets? Or is it a digital representation of some sort of credential or authorization? I'm particularly interested in how it fits into the broader cryptocurrency ecosystem and whether it has any specific use cases or advantages over other solutions. Your insights would be greatly appreciated.