A dyno chart is a two-dimensional representation of an engine’s horsepower and torque over the engine’s speed range, as measured by a device called a dynamometer. Most dynamometers either connect to the bike’s countershaft sprocket, or use a roller that bears against the rear tire (sometimes called chassis dynamometers).
Torque is a moment of force, acting at a distance, trying to rotate something. When you’re trying to twist the cap off a jar of pickles, you’re applying torque, whether the cap gives way or not—no movement is involved.
Horsepower is a measurement of actual work done in a given time. It was originally stated as the force needed to move 550 pounds one foot in one second, or 33,000 pounds one foot in one minute. Cycle World's dyno charts are calibrated in horsepower and pounds/feet of torque; countries using the metric system use kilowatts or PS rather than horsepower and newton-meters for torque. While the units of measure change, the fundamentals remain the same: you're trying to use these lines on a chart to understand an engine's output.
Power = force x distance ÷ time. In any measurement of distance and time, like a bike’s quarter-mile (400 m) times and top speeds, horsepower is going to be the determining factor.
But we don't ride our bikes at peak power or peak torque all of the time. What we need to do is look at the shape of a bike's horsepower and torque curves, and that's where a dyno chart is invaluable. To quote Cycle World's own Paul Dean: "Flat torque curves and wide powerbands make for great street motors; steep torque curves and narrow powerbands are usually best suited for racing."
Or, just shade in the areas under a bike’s torque and power curves with pencil. The more total shaded real estate under them, the more likely you'll enjoy the ride.