The Terraform merge function is a built-in operation that combines multiple maps or objects into a single map or object. It's available in versions 0.12 and later. Merging maps will prioritize the value from the last map provided as an argument if there are duplicate keys. To merge lists of objects, you can use functions like concat, for expression, or flatten. Merging lists of maps is similar to merging lists of objects. The merge function is also useful in applying merged tags to cloud resources.