Variables in build definitions are usually used to store secrets or values that can be used in multiple places/build definitions
When being logged in, click Variables on the left-hand navigation to list all available Variables. These are those you created and those others created and marked them Public.
If a public Variable from a different user and a Variable from you have the same name, your own Variable will have higher precedence.
Example scenario:
You can to create a build pipeline for a private GitHub repository, you need a
Personal Access Token.
You can create a Variable called e.g. github_token and use the corresponding
placeholder ${github_token} in your build definition, e.g.
name: vendor/repo-name
access_user:
access_secret: ${github_token}
branch: master