HIIT stands for High Intensity Interval Training. While the format varies tremendously, the idea is to do a set number of seconds of a cardiovascular exercise that significantly raises the heart rate, following by a period of rest and recovery. It is theorized that these kind of intervals more readily improve cardiovascular fitness and burn fat and calories. For example, the workout I did Monday morning was a 20 minute HIIT video I found on youtube. It included four rounds of five exercises. For each exercise there were 40 seconds of hard work followed by 20 seconds of recovery.
This HIIT-style approach can also be adapted to things like walking and jogging. For example, you can set a pattern in which you exert yourself and speed up to pace where it is difficult to hold a conversation for 30-60 seconds, then bring it down to a conversation pace for a 30-60 second recovery. Of course, always keep it within reason and train at your fitness level, but these little “pushes” can have big benefits to the human body.
If you want to read more about HIIT training I found this article gave a nice overview without getting too deep into the nitty gritty: https://www.self.com/story/what-is-high-intensity-interval-training-benefits