The document defines the derivative of a function and explains how to find derivatives. It states that the derivative of a function f(x) at a point a is defined as the limit as h approaches 0 of (f(x+h) - f(x))/h, if the limit exists. This provides a geometric interpretation of the derivative as the slope of the tangent line to the function f(x) at the point a. The document also defines left and right derivatives and notes that for a derivative to exist at a point, the left and right derivatives must be the same at that point. Additionally, it mentions that differentiability requires a function to be continuous at a point.