Embedded within strings in Terraform, whether you're using the Terraform syntax or JSON syntax, you can interpolate other values. These interpolations are wrapped in ${}, such as ${var.foo}.

The interpolation syntax is powerful and allows you to reference variables, attributes of resources, call functions, etc.

You can perform simple math in interpolations, allowing you to write expressions such as ${count.index + 1}. And you can also use conditionals to determine a value based on some logic.

You can escape interpolation with double dollar signs: $${foo} will be rendered as a literal ${foo}.