A VPC component that enables communication between resources in a VPC and the internet. For a subnet to be public, its route table must include a route to an Internet Gateway.
The prerequisite for public subnets on SAA-C03. A VPC can only have one IGW. For an EC2 instance in a public subnet to reach the internet, it needs: an IGW + a route in the route table + a public IP or Elastic IP.
Internet Gateway (IGW) is a concept that can show up on the exam. Check the full guide to see it in context.