for_each can be used to:

• loop through the items is a list (this article)
• loop through the keys and values in a dictionary

Here is an example of how for_each can be used to loop through the items in a list.

resource "aws_vpc" "my-vpc" {
  for_each = toset( ["foo", "bar"] )
  cidr_block = "10.0.0.0/16"
  tags = {
    Name = "my-vpc-${each.key}"
  }
}

You can have multiple values in a list, like this.

resource "aws_lb_listener" "listeners" {

  for_each = {
    "foo-listener" = [var.foo_port, aws_lb_target_group.foo-target-group.arn]
    "bar-listener" = [var.bar_port, aws_lb_target_group.bar-target-group.arn]
  }

  load_balancer_arn = aws_lb.application-load-balancer.arn
  port              = each.value[0]
  protocol          = "HTTP"

  default_action {
    type             = "forward"
    target_group_arn = each.value[1]
  }

  tags = {
    Name = each.key
  }
}

Often, you will be looping through a list that in one of your terraform.tfstate files that was generated from a data block or resource block.

Also be aware that if the items in the list are wrapped in double quotes, they will be interpreted as strings. There may be a situtation like this where you must NOT wrap the items in double quotes.

resource "aws_lb_target_group_attachment" "register-target" {

  for_each = toset([data.terraform_remote_state.ec2_instances.outputs.my_docker1_instance.id, data.terraform_remote_state.ec2_instances.outputs.my_docker2_instance.id])

  target_group_arn = aws_lb_target_group.my-application-load-balancer-target-group.arn
  target_id        = each.key
  port             = 12345
}

Let's say you have resource or data item that contains tolist, like this.

subnets = {
  "filter" = toset([
    {
      "name" = "vpc-id"
      "values" = tolist([
        "vpc-014d2fcfa335d3c01",
      ])
    },
  ])
  "id" = "us-east-1"
  "ids" = tolist([
    "subnet-03f11417780f6cdbc",
    "subnet-09b70fa463fcd4a19",
    "subnet-0f35c3586e5090314",
    "subnet-03c64e403dc5bf18f",
    "subnet-0316e4d9fcd4efccc",
    "subnet-05727079637285f90",
  ])
  "tags" = tomap(null) /* of string */
  "timeouts" = null /* object */
}

Here is how you could loop over each item in a tolist.

resource "aws_efs_mount_target" "my-efs-mount-target" {
  for_each = tolist(data.aws_subnets.subnets.ids)

  subnet_id = each.value
}

Or like this.

resource "aws_efs_mount_target" "my-efs-mount-target" {
  for_each = { for index, subnet in data.aws_subnets.subnets.ids: index => subnet }

  subnet_id = each.value
}