Let's pretend you have 20 minutes of free time. That's just enough time to watch a quick episode of one of your shows, do a face mask, or squeeze in a workout, but it's not enough time to go all the way to the gym or take a workout class. If you feel like working out, these random pockets of time are perfect for squeezing in a high-intensity interval training (HIIT) workout.
Basically, HIIT is a cardio training technique that implements quick bursts of exercise and intervals of rest. The idea is that by going back and forth between periods of exercise and rest, you're actually training your body to work more efficiently. Studies have shown that HIIT is more effective than sustained moderate-intensity exercise (like running or using the elliptical), and people tend to find it more enjoyable, too.
Pretty much any aerobic exercise can be done "HIIT-style," and you can also do strength-training exercises (like pushups, lunges, and planks) in HIIT intervals. The other cool thing about HIIT is that it just takes less time, so it's a great way to get in a full workout without feeling like you're cutting corners.