Humanity is the quality of being humane or distinctly human, and in writing it names the presence of a real person behind the words: a natural voice, warmth, and the sense of being addressed directly.
In William Zinsser's 📝On Writing Well, humanity is one of four principles of good nonfiction, with 📝Clarity, 📝Simplicity, and 📝Brevity. Zinsser treats writing as a personal transaction between one writer and one reader, and argues that what ultimately holds a reader is the writer's own personality rather than the subject alone. Writers who strain to sound literary, official, or impressive lose that voice; writers who relax and sound like themselves keep it.
The word derives from the Latin humanitas, from humanus, "human." In ordinary use it also means humankind as a whole and kindness toward others. In the craft sense it sits closest to "voice" and "warmth," and it stands against the impersonal, institutional register of much business and academic prose.
Be natural and be yourself; don't set out to write literature. Most writers freeze at the vision of their audience, but writing is a one-to-one experience, just you and the reader. What you write is who you are, so invite the reader along on the trip.