Your “threshold” referes to the level of intensity you can sustain when all the lactate being produced by your aerobic energy system is being reabsorbed by the  anaerobic system. a higher intensity would lead to build up of lactic acid and cause that all to familiar burning sensation. 

Threshold is often referred to as ‘sweet spot’ as it allows you to sustain a high level of output for a longer period of time. This provides a strong stimulus to the boy to adapt and get fitter.