Explore fundamental concepts in general relativity through this 59-minute lecture delivered by Olivier Sarbach from Universidad Michoacana de San Nicolás de Hidalgo as part of the Fields Institute's General Relativity Winter School. Delve into advanced theoretical physics topics as part of a comprehensive winter school program designed to provide in-depth understanding of Einstein's theory of general relativity. Gain insights from expert instruction within the broader context of the Thematic Program on Shocks and Singularities, focusing on nonlinear evolution equations in physical and life sciences. Build upon foundational knowledge through this second installment of the lecture series, which forms part of a structured educational program examining the mathematical and physical principles underlying spacetime, gravity, and relativistic phenomena.