A clock breaks time down into intervals of seconds, minutes, and hours. An interval is a distinct measure of time or the physical or temporal distance between two things.
When you are driving down the highway at 60 mph, you'll see distance markers at intervals of .1 miles. That means that every 1/10th of a mile, you will see one of these markers. Do the math and you'll see that these signs flash by you at intervals of 6 seconds. If your coach tells you to try interval training, he is telling you to do something like run for three minutes, lift weights for two, then run again for three. If he tells you do this again and again, you might want to end this interval of your life.