Apply and create it

Apply: make it real

apply carries out the plan. Terraform creates the server, then records it in state:

demo_server.web: Creating...
demo_server.web: Creation complete

Apply complete! Resources: 1 added, 0 changed, 0 destroyed.

Open the State Inspector afterwards. The resource is there with every attribute, including the computed ones the plan couldn't show: a real id (like demo_server-977a3db6) and a private_ip.

That record is how Terraform connects the demo_server.web in your code to the real server it created. Without it, the next run couldn't tell "this server is mine and already exists" from "this server still needs creating".