An apartment number for a device
Think of an IP address as a building address and a port number as an apartment number. A server can use one port for a website and another for email, sending each request to the right service.
Secure websites usually use port 443; other websites usually use port 80. Your browser normally chooses the port for you.
What an open port means
An open port means an app is ready to accept connections. That is normal for a public website, but a private service should not be open to everyone. A firewall can limit who can connect.