In Terraform, variables are defined and assigned in separate steps: your variables.tf (definitions) and your variables.tfvars (assignments).
In variable definitions you set things like their type, description and whether they have a default.
In variable assignments you set their actual value.
Stacks removes the need to define variables and defines any undefined variables for you.
All you need to do is nothing. You can "forget" to define your variables with Stacks.