Port
A "channel" on a server — a numbered entrance for a specific service.
An IP address is like a house address; the port is like the apartment number. A server can simultaneously have a web server on port 80, a mail server on port 25, and an SSH server on port 22.
Common: 22 (SSH), 25 (SMTP), 53 (DNS), 80 (HTTP), 443 (HTTPS), 3306 (MySQL), 5432 (PostgreSQL), 6379 (Redis), 22000–22999 (often blocked by strict firewalls — try to stay under 1024 if possible).