What is Terraform?
Terraform is an open-source DevOps tool that provides infrastructure as code capabilities. It allows you to create declarative configuration files that can use various cloud API's to create, update and delete your infrastructure.
Now you've had a high level overview on what it does lets get and install it on Ubuntu 18.04.
HashiCorp provide their own repositories for installation on the main flavours of Linux.
curl -fsSL https://apt.releases.hashicorp.com/gpg | sudo apt-key add - sudo apt-add-repository "deb [arch=amd64] https://apt.releases.hashicorp.com $(lsb_release -cs) main" sudo apt-get update && sudo apt-get install terraform
Running the above commands will add the gpg key to your key store, add the repository to your list that apt gathers software from and install the latest version (1.1.7 at time or writing).
When completed you can run
terraform --version to see version you have installed.
Now that you have the application installed locally give it a go following the tutorials provided by the HashiCorp Learn website.