The lesson focuses on diabetes, specifically Type 2 diabetes, as well as other forms such as Type 1 and gestational diabetes. The teacher explains how glucose and insulin work in the body, highlighting that Type 2 diabetes occurs when there is insufficient insulin to manage blood sugar levels, leading to high glucose in the bloodstream. The lesson also touches on risk factors and the development of diabetes, stressing the importance of understanding these conditions for effective energy utilization in the body.