Open port check
Is a TCP port open from the internet? One host, one port.
Open, closed and silent ports
A port is a numbered door on a computer. Open means a program is listening and accepted the connection. Closed means the computer answered that nothing is listening. Silent or filtered means there was no answer, usually because a firewall drops the connection.
Ports 0 to 1023 are the well-known range: 80 HTTP, 443 HTTPS, 21 FTP, 25 SMTP, 110 POP3 and 143 IMAP. Port 465, 587 and 993 carry encrypted mail.
This tool makes a full TCP connection (the same thing a browser does) to one port. Other methods exist, such as a ping or a half-open SYN scan, and TCP and UDP are different protocols; we only use the full TCP connect.
Why close unused ports
Attackers scan the internet for open ports to find services to break into. Close any port you do not need, restrict the rest to the addresses that need them, and keep the software behind them up to date.