This lecture is the second in a series on High-dimensional Expanders (HDXs) presented by Madhur Tulsiani at the International Centre for Theoretical Sciences. Explore the mathematical foundations and applications of HDXs, which generalize expander graphs to higher dimensions such as hypergraphs and simplicial complexes. Part of the "ICTS Workshop on HDXs and Codes" program organized by Irit Dinur, Venkat Guruswami, and Prahladh Harsha, this 75-minute lecture contributes to a broader two-week program connecting High Dimensional Expanders with Error-Correcting Codes. The workshop brings together researchers in computer science, mathematics, and electrical engineering to explore recent breakthroughs in these interconnected fields that have significant applications in communication technology and theoretical computer science.