It’s a new thing. Not everyone really believes such thing exists. You could say that it is a person, who plans the structure of a website in a long term (for a year, two, three).
Do do it good (plan the structure of a website) you have to imagine very well WHO are you writing to and WHAT do you want to say to them.
The most important thing to distinguish when searching for such people is that they are NOT copywriters/ text writers. They plan website, but it does not mean that they will have to write the text themselves.
Usually this thing is done for larger websites.
The other important thing here is that anyone can SAY they can do it – but it is very hard to ACTUALLY do it. So in this case I need some proof of experience doing EXACTLY THAT - Content Strategy (not writing, not copywriting, not article writing, not data entry or anything like that).
I need some proof they were PLANNING website or web app strategy, maybe portfolio item or something like that.