An elastic network interface is a logical networking component in a VPC that represents a virtual network card.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html

Each instance has a default network interface, called the primary network interface. You cannot detach a primary network interface from an instance. You can create and attach additional network interfaces.

Each network interface is attached to a network card.

Instances with multiple network cards provide higher network performance, including bandwidth capabilities above 100 Gbps and improved packet rate performance.