In this episode Sam and James discuss lighting and vision rules in D&D. You can find an explanation of these rules in the Player's Basic Rules D&D PDF on page 65 or in the Player’s Handbook on page 183.
D&D 5e Basic PDF: http://dnd.wizards.com/articles/features/basicrules
James’ blog: http://www.worldbuilderblog.me
Sam’s blog: http://www.rpgmusings.com