Simplicity is the quality of being plain, uncomplicated and free of unnecessary parts; in writing, it means stating an idea with no more words or ornament than it needs.
In William Zinsser's 📝On Writing Well, simplicity is one of four principles of good writing, with 📝Clarity, 📝Brevity and 📝Humanity. Zinsser calls clutter the disease of American writing. Most adverbs and adjectives are unnecessary, and hedging qualifiers such as "sort of" dilute a writer's power. Every component of a piece should be doing new work, short sentences are better than long ones, and a writer should learn to cut and to enjoy cutting.
Zinsser's illustration is an airline pilot who announces that "we are presently anticipating experiencing considerable precipitation" and would never dream of saying "it may rain." The inflated version sounds official but says less, and more slowly, than the plain one.
Cutting writing is like cutting 📝Deadwood.