I assume they are talking about something like https://github.com/cdr/code-server .

It’s vscode in your web browser. Run it locally in your dev environment and setup your web server to proxy vscode.your domain.tld to it and boom, you have vscode running in your dev environment that anyone with a link can access.

The fastest way to set it up with tls+auth is probably something like https://hub.docker.com/r/linuxserver/code-server + caddy configured with basicauth.