In this episode James and Sam discuss basic spellcasting rules in D&D. You can find an explanation of these rules in the Player's Basic Rules D&D PDF on pages 78-81 or in the Player’s Handbook on pages 201-205.
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