Networking started in the seventies.
Companies like IBM and DEC made their own network protocols. (IBM used to be the biggest computer company by far. And the Digital Equipment Coorporatation was the 2nd largest. DEC went bankrupt in the nineties). Apple made AppleTalk. Novell made IPX. Etc.
But this meant that computers from brand X could only talk to other computers from brand X. An IBM computer could not talk to an Apple computer or a DEC computer. Because they spoke different protocols. Also, the protocols themselves were owned by those companies. That is why we call those protocols "closed". If you were not IBM (or an IBM customer) you could not use their protocols. You weren't even allowed to know how they worked. You could not build your own network drivers or network equipment to interoperate with those computers.
And then there are open protocols.
The OSI family of protocols, and the TCP/IP family of protocols are the most famous ones. Everybody can help define those protocols. (Although the OSI protocols mostly got input from large Telecom companies). Everybody can see in detail how those protocols work. Everbody is allowed to build equipment that interoperates with other OSI or TCP/IP computers. That's why they are called open.