A flywheel is essentially a heavy disc capable of holding large amounts of energy. By spinning it faster and faster, you can store your effort to use later.
This has a wide range of industrial applications, like smoothing out the output of an energy source or applying a much stronger, sudden force on something than the energy source could deliver on its own.
Flywheels are so effective, you can even technically use them to power a bus, and in the 1940s, the Swiss did exactly that when they invented the gyrobus. It was just like a normal bus, but instead of running power lines along its entire route or putting a polluting internal combustion engine in it, they added a giant, steel wheel that could be respun at each stop.
Basically, it had a clever, relatively low tech battery.
Unfortunately, running a bus this way is a little impractical. A giant, spinning wheel is energy intensive, changes the physics of turning the vehicle it's inside, and puts passengers near something moving at dangerous speeds.
But flywheels are a useful mental model for thinking about a whole range of things.
A body in motion tends to stay in motion. The challenge is getting it started, because maintaining it is relatively easy, since it requires less energy. If you've ever rolled something heavy, you know how much easier it is to keep it moving than to constantly stop and start.
If you're running a successful business, you can create a flywheel effect by encouraging customers to refer other customers to you through a referral program. This creates sustainable, forward momentum. As you acquire more customers, they tell others, who in turn purchase your product and spread the word as well.
In our personal lives, a flywheel is a metaphor for wellbeing. If you get enough sleep, you're more likely to have the energy and willpower to eat healthy and exercise. This gives you more energy and improves the quality of your sleep.
And so the cycle continues, as long as you keep the wheel spinning.