Terraform 0.12 is a major update that includes dozens of improvements and features spanning the breadth and depth of Terraform’s functionality. First-class expression syntax: express references and expressions directly rather than using string interpolation syntax. Generalized type system: use lists and maps more freely, and use resources as object values.
Iteration constructs: transform and filter one collection into another collection, and generate nested configuration blocks from collections. Structural rendering of plans: plan output now looks more like configuration making it easier to understand. Context-rich error messages: error messages now include a highlighted snippet of configuration and often suggest exactly what needs to be changed to resolve them.
Source: hashicorp.com