In this episode James and Sam discuss the Initiative mechanic in D&D. You can find an explanation of this rule in the Player's Basic Rules D&D PDF on page 69 or in the Player’s Handbook on page 189.

Relevant Links:

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