under the hood

adjective

a metaphorical area that contains the underlying implementation of something - e.g. a piece of hardware, a piece of software, an idea, etc.
Let's now look under the hood to see how the software goes about transmitting data so quickly.

To understand how it really works we need to look under the hood.

Does your excursion call for more power than the 173-horsepower four-cylinder under the hood?

What does "under the hood" mean in programming?

解答1

Think of an automobile. We can (and often do) drive an automobile without understanding how it works. Most people never look at the engine of the car, which lives under the hood (we call it the hood in America, but for example, British folks call it the bonnet).

In addition to the engine, there is a transmission, which takes the power generated by the engine and transmits it to the drivetrain, which ultimately takes that power and turns the wheels. Most people don’t understand any of these details, nor do they need to in order to operate an automobile.

As a programmer, it’s often helpful to know what’s happening under the hood, because it gives you insight into things you cannot see.

解答2

The American expression “under the hood” is an allusion to an automobile. The “hood” refers to the hatch covering the engine compartment wherein one finds the engine and internal components that make up the bulk of the functional electronic and mechanical components of the vehicle.

In the context of something like a computer, an American would understand it to refer to the working parts inside which you don’t usually see, but you know are there, doing things, complex things, important things. In the case of software, it refers to services or subroutines where the implementation is not known to you, but it works and does what it is supposed to (most of the time).