The TCP monitoring check is a part of the ClouDNS motoring service, that makes a TCP connection to the given IP address on the chosen port number. If the connection is unsuccessful, the check is marked as DOWN.
TCP is used to provide a reliable, ordered data delivery between two applications. It is a delivery service that guarantees that all bytes received will be identical and in the same order as those sent.
TCP is important because it establishes the rules and standard procedures for the way information is communicated over the internet. It is the foundation for the internet as it currently exists and ensures that data transmission is carried out equally. By enabling TCP Monitoring you are checking that a service is accessible on a given port on the network device by establishing a connection before data transmission begins.
Each Monitoring check provides debugging option which you can use to find out where the problem comes from using Traceroute information - The traceroute command is used to determine the path between two connections. Often a connection to another device will have to go through multiple routers. The option can be found in the Monitoring check settings page on the right side of the "Uptime changes section".