A flywheel is a heavy wheel on a spinning shaft, put there to keep a machine turning at a steady speed. Many engines do not push with the same strength every moment — a car engine, for example, goes bang, then a pause, then bang. The heavy wheel keeps spinning through the weak moments and gives energy back when the push is thin, so the machine does not jerk. It stores turning force and smooths the ride.

Fly here means keep moving, keep spinning — not the insect, and not flying in the sky. Wheel is just wheel.

You will also meet the idea in a gym. A spinning bike has a heavy wheel at the front; that wheel is a flywheel too, used to make the pedalling feel smooth and hard. Though actually, in Taiwan the word 飛輪 often makes people think of that gym class first. In a science text or a car workshop, look for the heavy wheel inside the machine. Same object-idea, two worlds.