Your gauges help you to control your Orbiter. Where you touch the screen determines both which direction you aim and how fast you accelerate.

1.  If you touch inside the Base Line (the green circle), you'll thrust at less than 1 Earth Gravity (9.8 m/s^2). This will use less fuel, but you'll move more slowly.
2. For more speed, touch outside the Base Line. You'll thrust at more than 1 Earth Gravity (but you'll use more fuel).
3. To move really quickly, touch outside the Red Line, for more than 3Gs of thrust. But besides burning fuel at an incredible rate, this also has a chance of causing your engine to overheat.