The lesson discusses puberty, focusing on the physical and emotional changes that occur in boys as they grow up. It highlights differences between a boy and his reflection in the mirror, illustrating changes such as increased height, facial hair, and the development of sexual characteristics. Puberty typically begins between the ages of 9 and 14 and involves the transition from childhood to adulthood, including the development of new feelings and interests. The session also mentions that while these changes can be awkward or surprising, they are a normal part of growing up.