This varnish clogs the fuel metering ports and causes the engine to run erratic or not at all. Stabil is a popular brand. Always put the engine in storage with at least a half tank of fresh treated fuel in it. Run the engine for 10 minutes after treating the fuel, so that the stabilizer gets into the entire fuel system. Start the unit up every month or two to keep fresh fuel in the carburetor during the season.
Fuel does evaporate from the carburetor.
The important thing to understand is that gasoline does not do well when stored in small quantities or open sunlight. If you don't use much fuel during the season, then try to keep what you have fresh and in a cool shaded area. Dump it in your car and get a fresh can every month or so. Most small engine shops sell gasoline stabilizer, as do hardware stores, etc. If you already have the stale fuel problem, then start by cleaning as much of the old fuel out as possible, then with fresh fuel in the tank, try starting it.