Your GraphCMS endpoint is the API URL where all of the data for your project is accessible.
"GraphQL is typically served over HTTP via a single endpoint which expresses the full set of capabilities of the service." -- graphql.org
You can find your API endpoint on your project dashboard and settings. You are also able to test it out in your project's API Explorer.
When you create a new stage, your content will be served to a new endpoint. This URL can be found in the same place as the master endpoint, in settings or on the dashboard, in the dropdown under the respective stage name. Learn more about stages here.