What is "threshold" training?

On today's episode of Run with Coach Parry we're talking some running lingo. Specifically, what is "threshold"? We're constantly hearing people talk about running at / below / above threshold - what does this mean and how important is it for my training?

Coach Parry gives insight into the different terms used and the physiology behind this concept. He also advises on what portion of your training should be done at this intensity.