Port
In plain terms
A numbered endpoint identifying a specific service on a host.
Going deeper
A port lets one machine run many network services by directing traffic to the right one via a number alongside the IP. It distinguishes services on the same host.
Under the hood
Well-known ports (80 HTTP, 443 HTTPS, 22 SSH) are standardized; combined with an IP they form a socket.
Examples
Port 443 (HTTPS), 22 (SSH), 80 (HTTP), service routing, firewalls.
Think of it like...
Like apartment numbers at one street address directing mail to the right unit.