Stacks is an orchestration layer for Terraform in HCP Terraform. You specify components of your infrastructure (e.g. frontends, backends) and deployments (e.g. development / production). Stacks runs each component / deployment combination as their own Terraform Root Module. Components can be connected, deployments are isolated.
You can find more information in the official blog post and the official documentation.
For a deeper technical comparison of the Stacks and Module runtimes, see Terraform Stacks vs Modules: How the Two Runtimes Differ.
