What is the Cosmic Redshift?

Edwin Hubble discovered the expansion of the Cosmos by seeking the distance to the Andromeda Galaxy. What is the cosmic redshift and how was it found? This is part of my intro Astronomy class I taught at Willam Paterson University and CUNY Hunter. Supplement the videos with "OpenStax Astronomy" https://openstax.org/books/astronomy/pages/26-thinking-ahead 26: Galaxies