That sounds correct. It is supposed to wait. As soon as you start the workflow manually, it will wait till it receives a call from Shopify. So you would start the workflow and then do the action you are listening for (like for example update an order). n8n will then receive the call and displays you the data it did receive.
If you run it locally for testing it is enough to use the tunnel as written in my previous post. If you run it in production on a proper webserver, you would need an own subdomain. Theoretically would also be an IP enough. That would however cause problems with most third party trigger nodes (probably also Shopify) as they do not allow to register webhooks on IPs. They only allow a proper domain and most time does it also have to run via https.