A turbo, short for turbocharger (which is also short for turbosupercharger) is a device that adds power by forcing more air into your engine.
Think of it like a water wheel where the water is the high-speed air flowing out of the engine’s exhaust. A turbo has a fan that is spun at extremely high speeds by that exhaust. That fan turns another fan, and that second fan sucks air into the engine and packs it into the cylinders.
That extra forced air can be combined with more fuel, so it makes your small engine act like a big one — or a big one act even larger.
Because it only forces in that extra air (called boost) when you need it, it lets automakers have a small engine for fuel economy when cruising, but get the benefits and power of a large engine when you hit the gas pedal. For more details, check out the Toyota Engines 101 videos.