Basically,
- Unless they stop calling out Trump, they lose Section 230 protections. Section 230 is what allows a platform like Twitter or Hacker News to do limited good-faith moderation and not be deemed editorially responsible for every piece of nonsense someone tweets/posts that they don't get around to moderating. Without it, you either have to moderate heavily and get everything right, or not moderate at all. (See https://en.wikipedia.org/wiki/Section_230_of_the_Communicati... - pre-Section 230, CompuServe did no moderation and was ruled not responsible for libelous user comments, and Prodigy did and was ruled responsible for something they failed to moderate, and people went "Wait a second, this seems like the opposite of what we want.") The executive order says, roughly, that the behavior Trump doesn't like isn't permissible under Section 230 and makes you editorially responsible for everything, so if you enjoy your Section 230 protections, you can't do that.
- The executive branch is discouraged from spending funds (i.e., buying ads) on platform that Trump doesn't like.
- The FTC will go after these platforms for unfair trade practices.
- The US attorney general will team up with state attorney generals to see if there's any state laws they can wield against these platforms too.