Oxygen in the air is what burns in the combustion chamber, which is why nitrous (oxygen rich) makes so much power when you inject it into the mix.
The more fuel (a catalyst for combustion) you introduce into the mix, the better combustion will be, hence you hearing that people raise fuel pressure and gain power.
However, there is a point that the air:fuel mixture can be too rich with fuel (rich condition). When this point is crossed, the fuel will start displacing the optimal amount of air in the mixture and you will have less oxygen than is optimal for complete combustion and you will lose power.
So, if oxygen is what you need to make power, why not run really lean? Because fuel also cools the combustion process down a bit so things don't overheat in all the explosions going on inside the combustion chamber. If you run too lean (of fuel) too long, you can blow your motor and/or induce predetonation among other things causing you too rebuild the motor.
Every type of fuel has an air:fuel ratio that is considered stoichiometric. For gasoline, this number is 14.7:1. Most EFI computers try to run this AF ration at idle and get lower (richer) as you give throttle (introduce more air) to make good power and keep things cool. Ethanol is around like 9:1 if I am not mistaken but it could be a little higher. Don't worry about that though. The AF ratio you should want to make good power safely on a naturally aspirated engine is 13-13.5:1. If you run a lot of fuel pressure and it is causing you to run 11:1 (ideal for boosted applications) and you are NA, then you will gain power by lowering the fuel pressure or getting a tune/burn/flash, etc. If you are running over 13.5:1 when on the throttle, you will both gain power and safety by rasing fuel pressure (or getting a tune/burn/flash, etc.).
So in order to know what your AF ratio is you need to get a WIDEBAND O2 sensor/meter or get on a dyno that has one. This is the only thing that can effectively tell you what you need to do.
