It’s everything. Those letters and numbers, words and punctuation marks create all meaning, understanding and value.
Designers that don’t know better tend to dismiss writing as text or content. Which is understandable because a lot of poor-quality writing is indistinguishable filler.
But words are code, not content. Words are code that tell our ideal reader whether to pay attention to us or not, whether to listen to us or not, and whether to buy from us or not.