They exercise for a short period at a very fast pace, rest until they recover and then repeat these fast bursts until their muscles do not recover from the hard bursts. A marathon runner may run 6 repeat very fast half miles at 2 minutes each with a quarter mile jog between each fast run.
Interval training teaches their brains to coordinate their muscles at a very fast pace.
The principle behind interval training is to move very fast so that lactic acid starts to accumulate in your muscles which makes them start to burn and hurt. Your muscles need energy when you exercise and they get their fuel from fat and sugar in your muscles and bloodstream. The sugar and fat are broken down through a series of chemical reactions each of which release a little energy at a time. If there is enough oxygen in your bloodstream, the sugar and fat are broken down into carbon dioxide and water which are blown off from your lungs when you breathe. When you exercise so vigorously that you can't get all the oxygen that your body needs, the sugar cannot be broken down to carbon dioxide and water and the sequence of reactions stops and a chemical called lactic acid accumulates in your muscles and spills over into your bloodstream.