This lecture focuses on the theory and methods for solving separable differential equations. The instructor begins by establishing the existence and uniqueness of solutions before introducing the concept of separable variables. The lecture defines the types of functions involved in the problem and presents the necessary conditions for the existence of a unique local solution. The instructor explains how to separate the variables and integrate to find the solution. Key concepts include the continuity of functions and the importance of ensuring that certain functions do not vanish. The lecture also covers the construction of the solution using integrals and demonstrates how to verify that the constructed function satisfies the original differential equation. The instructor emphasizes the uniqueness of the solution derived from the properties of monotonic functions and their inverses. Finally, the lecture sets the stage for practical examples in subsequent videos, illustrating the application of these theoretical concepts in solving specific equations.