Imagine you have a toy car and you want to race it around a track. Sometimes you want to race the car really fast and sometimes you want to race it a little slower. Sometimes you race the car for a short time and sometimes you race it for a long time. The intensity-duration-frequency curve is kind of like a map that shows you how much you can race your toy car before it gets tired and can't go as fast anymore.
Intensity means how fast you are racing the car. High intensity means you are going really fast and low intensity means you are going slower.
Duration means how long you are racing the car. Short duration means you race the car for only a little bit and long duration means you race the car for a long time.
Frequency means how often you race the car. Do you race it every day, once a week or only once in a while?
All of these things (intensity, duration, and frequency) can affect how well your toy car works. The intensity-duration-frequency curve helps you figure out how much you should race your car so that it works the best. For example, if you race your car really fast for a long time every day, it might get really tired and break down. The curve can tell you how much is too much so that you don't break your toy car. This is important because the curve can also help people figure out how much exercise is good for them without overdoing it.