First off, let's make sure you are clear on what a protocol is. By definition, a protocol is the rules of communication. If you are travelling in a foreign country, you need to be aware of the customs of that country. A gesture that may seem friendly or insignificant to you may actually be considered an insult in other parts of the world. Awareness of protocol can save you the embarassment of miscommunication.
Protocols are even more important if two computers wish to exchange information.
Amazingly, computers communicate by subtly changing millions of electrical pulses, light pulses, or radio waves per second. Both computers need to be using the same protocol, or set of rules, to correctly interpret which of these pulses represent the address of the computer to receive the data, the address of the computer that sent the data, the data itself, and confirmation that the data received was the same data that was sent. Proprietary means that in order to exchange information with another computer, it has to be running the same hardware and the same version of the same operating system, which was provided by the vendor of the hardware. For example, the port number for Telnet is 23 and the port number for http is 80. The other computer will realize that I wish to use the rules for Telnet, which are very different than, say, the rules for surfing or checking my e-mail.