Metcon is short for Metabolic Conditioning. This is how we often deliver cardio training or circuits. The aim of a Metcon is to use weights and/or body movements to expend energy (burn calories), increase your strength and cardiovascular endurance. There are various structures that we use in order to achieve this, and as a means to add variety to your workouts.
Aside from being a useful tool for fat loss and fitness related goals, a metcon is far more interesting than the typical steady state cardio that is often seen in gyms.
While running on a treadmill for long periods of time can be effective in burning calories, it can be fairly dull. A metcon can fulfil the same calorie burning benefits in an infinite number of ways.