The terraform init command is used to initialize a working directory containing Terraform configuration files. This is the first command that should be run after writing a new Terraform configuration or cloning an existing one from version control. It is safe to run this command multiple times.
This command is always safe to run multiple times, to bring the working directory up to date with changes in the configuration. Though subsequent runs may give errors, this command will never delete your existing configuration or state.
terraform init command does -
* Copy a Source Module
* Backend Initialization
* Child Module Installation
* Plugin Installation
https://www.terraform.io/docs/commands/init.html