Differentiation is adjusting and modifying what skills and concepts students learn, what materials they use, and/or how their learning is assessed based on the needs of the students.
Our students are not all the same, so we cannot expect that teaching a lesson in one way will reach every student. Our students are unique individuals with their own prior learning and their own learning preferences.
An effective teacher makes the changes needed to reach every student where they are and to guide them to an understanding of the skills and concepts we want them to learn.