Words in a product are like the holds on a rock climbing wall. Language that’s “easier to grasp” makes a product easier to use. Content design is the principled use of language to help people do things.
That’s how I think about it at least. Some definitions by people smarter than I:
Content in UX/UI design is the language that appears in a user interface. Content design, sometimes called UX writing, is essentially designing with words.UX Design Institute
Content designers design in words, concepts, systems and terminology, voice and tone, and know how much these things matter in solving problems for the people who use our products around the world.Elizabeth Carr
Content design… involves establishing good relationships, taking the time to get to know the product domain, and working out how you can contribute the most value, based on that particular project, context, and phase.Tom Waterton
Content design is the principled use of language to help people do things. It is also called UX writing or UX content strategy. (See Nomenclature problem)
How a product sounds affects how easy it is to use (usability), who can use it (accessibility), and how well it does what it’s supposed to (efficiency). Content design is about improving these three vectors and so much more.