This lecture covers the concepts of compressibility and thermal expansion in materials. It explains how the volume of a body changes with pressure variations and how dimensions change with temperature. The instructor discusses the relationship between Kelvin and Celsius scales, the ideal gas law, and the coefficient of compressibility. Additionally, the lecture explores how the dimensions of a body change with temperature variations, introducing the coefficient of thermal expansion. Practical applications such as thermostats and electrical contacts are also discussed, along with examples illustrating the concepts of compressibility and thermal expansion.