It is a simple free port forwarding checker that can be used to find out whether a particular port is open or blocked by a firewall.
Although port numbers are essentially abstract certain applications occupy common port numbers. I have listed the most popular ones above which you can click to check against your host.
On a router it is typical that most ports are blocked to prevent malicious activity. Your router is essentially a proxy between you and the internet so the act of allowing a program or game using port or range of ports to pass through is known as port forwarding.
TCP Port Service Multiplexer (TCPMUX)
Message Send Protocol (MSP)
SSH Remote Login Protocol
Simple Mail Transfer Protocol (SMTP)
Host Name Server (Nameserv)
Login Host Protocol (Login)
Domain Name System (DNS)
Trivial File Transfer Protocol (TFTP)
SNA Gateway Access Server
Simple File Transfer Protocol (SFTP)
NetBIOS Datagram Service
Interim Mail Access Protocol (IMAP)
NetBIOS Session Service
Border Gateway Protocol (BGP)
Gateway Access Control Protocol (GACP)
Internet Relay Chat (IRC)
Directory Location Service (DLS)
Lightweight Directory Access Protocol (LDAP)
Novell Netware over IP
Simple Network Paging Protocol (SNPP)