> What exactly does "hijack" mean though? I don't speak IRC so it's not clear what they're actually doing to the channels.

In Reddit terms, imagine the Reddit administrators coming to your subreddit, deleting the moderators and then installing their own.

Administrators always had this power. Sometimes, this power was used out of necessity. But given how much this power was used in FreeNode in the past week, a lot of the community has lost faith in the administrators.

But yeah, if you control the server (the nature of being an administrator), you can do these things rather easily.