The most thorough explanation of what the Anonymous operation "#keepthep0rtalsopen" really is:
https://tilde.camp/openbook/IMPORTANT.txt
This is one of the earliest videos introducing #keepthep0rtalsopen:
https://www.youtube.com/watch?v=6-j4FKongnE
"#thep0rtals" are essentially referring to each instance of Openbook. There are supposed to be many different Openbook websites, each with their own purpose or collective identity, but all of them allow anyone to post anonymously. Although Openbook is just a website software, and not a decentralized software, the idea was that the website could still be decentralized just by having lots of different people putting up an instance of Openbook on their own servers, so that the website has lots of duplicates that all link to each other. This means, even if any of these websites go down for some reason, there will always be this whole network of nearly identical websites that allow people to continue communicating anonymously. This is especially important for people who are not as tech savvy, and don't know how to use the Tor network and find IRC channels and all that crap.