If you're asking what happens when you press the gas pedal while the car is in neutral, not much will happen. The engine will rev up, but since the car isn't in gear, it won't go anywhere. You might see a small amount of movement from the car if it's on an incline and gravity is pulling it downhill, but other than that, pressing the gas in neutral won't do much.
However, if you're talking about what happens when you accidentally press the gas pedal while the car is in gear and your foot is off the brake, that's a different story. When this happens, called rolling away,the car can lurch forward unexpectedly and cause problems. If you're parked on an incline, rolling away can be especially dangerous because the car could gain enough speed to roll into traffic or hit something else. So if you're ever getting into or out of your car and think there's a chance your foot might slip off the brake and hit the gas instead (it happens!), make sure to put it in park first.