This lecture introduces the concept of adjoint functors in category theory, focusing on the characterization of adjunctions. The instructor explains the conditions for a pair of functors to form an adjunction, emphasizing the existence of natural isomorphisms between them. The lecture covers the demonstration of mutual inverses and the need for specific applications to illustrate the concept. Additionally, the slides delve into the verification of triangular identities and the implications of natural transformations. The instructor highlights the importance of checking these identities to complete the characterization of adjoint functors.