When you connect to a server, you send traffic to an IP address. That is like a street address with a letter. But when you send a letter in the mail, more than one person could live at a house, so you also include a name (or port number when sending to one service such as web traffic to a server that does many things). Usually that name corresponds to a person living there, like if you send a letter to John even though Sarah also lives there.

That is usually how it works, but not always. John and Sarah may have agreed that all letters should be named backwards, so John opens all letters to the name Sarah, and Sarah opens all letters to the name John. This would confuse anyone trying to send a letter to John unless John told them to use the name Sarah when sending a letter. Both could also agree that if a letter shows up for Bob, Sarah should open those as well (only Sarah's best friend would know to use the name Bob).

These exceptions are usually people either trying to obfuscate what is running on a port to pretend to be something different, it trying to make it hard to find the port associated with the server.