Onboard computers generally control modern engines, and they constantly adjust the amount of fuel and air that enter combustion chambers, when the mixture is ignited, how far the valves will open, etc. The onboard hardware that manages these tasks is known as the Engine Control Unit.
The software in the Engine Control Unit works to find a balance between power, fuel economy, emissions, and longevity. Many road cars almost never pursue maximum power output at the expense of these other objectives—but your car is capable of producing more power by making compromises elsewhere.
When you add a performance chip, you replace the car’s Engine Control Unit with a new one—and this one uses different software. Long story short, that’s where the increased power comes from!