I am currently studing them but am not quite fully understanding how things work. Specifically I don't understand how the vairables below -- such as http_host, $remote_addr, etc -- get defined.

server {
  location / {
      proxy_set_header Host $http_host;
      proxy_set_header X-Forwarded-Host $http_host;
      proxy_set_header X-Real-IP $remote_addr;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_pass http://localhost:5000;
    }
}