Differential (dy) Notation and Intuition - Expii
A differential dx is not a real number or variable. Rather, it is a convenient notation in calculus. It can intuitively be thought of as "a very small change in x", and it makes lots of the notation in calculus seem more sensible. The derivative dy/dx, for example, captures the following intuition: "if we add a very small change to x, what is the very small change that y experiences?".