This lecture is the first in a series on High-dimensional Expanders (HDXs) delivered by Madhur Tulsiani at the International Centre for Theoretical Sciences. Explore the foundational concepts of HDXs, which generalize expander graphs to higher dimensions such as hypergraphs and simplicial complexes. Part of the ICTS Workshop on HDXs and Codes, this lecture contributes to a program bridging High Dimensional Expanders and Error-Correcting Codes—two fields experiencing significant recent breakthroughs. The workshop includes a week-long school for graduate students and postdocs, a three-day technical workshop, and a special two-day event celebrating Jaikumar Radhakrishnan's contributions to combinatorics, complexity, and information theory.