As the mower sits through the winter, fuel remaining in its engine will decompose, "varnishing" the carburetor and causing difficulty when you try to start the engine in the spring.
If you've added stabilizer to your fuel to keep it fresh longer, then fill the gas tank to the top with more stabilized fuel and run the engine briefly to allow it to circulate. If not, wait until the tank is nearly empty from use and run the engine (outdoors) to use up the remaining fuel. Check your mower's manual for other cold-weather storage steps.
Avoid costly spring repairs and inconveniences with this simple and quick step!