The Hashicorp Configuration Language (HCL) used by Terraform includes several built-in functions that can manipulate and transform values within an expression. Two such functions are join and split, which work with strings. Join produces a string by concatenating elements of a list using a specified delimiter, while split separates a single string into a list using a given delimiter. These functions can be useful in various scenarios, including working with configuration files for devices or appliances, extracting parts from URLs, and more.