What does protocol -1 on AWS security group egress mean?

I understand the egress property on an AWS security group controls the outbound traffic, but does anyone know what the protocol of -1 means?

resource "aws_security_group" "elb" {
  name = "example-elb”
  ingress {
    from_port = 80
    to_port = 80
    protocol = "tcp”
    cidr_blocks = [" 0.0.0.0/ 0”]
  }
  egress {
    from_port = 0
    to_port = 0
    protocol = "-1"
    cidr_blocks = [" 0.0.0.0/ 0"] }
}