The internet is open to everyone and is for everyone, regardless of who that person may be. Even those who may not have personal access to a device that works with the internet could go to a place such as a library in order to use the internet.
What it means to create something that is “open” is it has the ability to take in contributions from people anywhere and at any time. For instance, people that contribute to the internet’s data help in its improvement and advances. The benefits of leaving a project open are that you can get more data and assistance without even asking for it. There’s a constant flow of information to help the system.
How the internet is open is in the way it can connect people through various mediums such as social media, links, webpages, etc. People can share media with each other using the internet making it open.
Making something “accessible” means that everyone is able to use a product or media with ease. You would want to make something accessible due to moral, legal, and business reasons.
How you make the internet more accessible is by using closed captioning, larger fonts, breaking up text into subsections, using hyperlinks, having a text-to-speech reader, etc. There are many different tools nowadays to ensure that electronic media is able to be consumed by anyone regardless of disability.
One resource I recommend to learn more about how to be accessible online is this article by Interaction Design Foundation titled “3 Reasons Why Accessible Design Is Good for All.”