Although designers have been involved in UX writing for quite some time, UX writing as its own discipline is an emerging field in the world of design.
UX writing is about… you guessed it… a product’s user experience and consists of the words we see or hear when using a product. It focuses on the words, also known as the strings or microcopy, as a part of your design. Just as a film is incomplete without sound, a design is incomplete without microcopy.
Great UX writing unites a product’s design and functionality with the text in a way that helps users understand what to do. UX writing guides users through an experience. It’s the text for headers, labels, paragraphs, buttons... it’s everywhere, which is why it’s so important!
In a nutshell, good copy consists of two attributes: usefulness and clarity. But there’s much more to it than that. Voice & tone, brand persona, conversational language, emotion, language translation, and many other factors come to play when writing for design.
Just like UX as a whole, UX writing keeps users at the center of the process. It’s a best practice to work directly with users to get feedback on the writing. It’s iterative. In essence, users are "co-creators" of the copy. Sometimes designers assume that if the text is clear to them it’s also clear to users. Well, you know what happens when you assume… Just because copy is clear to you, doesn’t mean it’s clear to someone else. People can easily get tripped up with even the simplest of words. Don't assume anything until users have interacted with your design.
A great resource to get started with improving your UX writing skills is Strategic Writing for UX by Torrey Podmajersky.
P.S. In case you were wondering, the link above is not an affiliate link.