Exposure is the heart and the art and the act of taking a photograph.
In film cameras, the film must be exposed to light, the light causes a chemical reaction to occur which captures and records the picture.
In a digital camera, the action is the same except, instead of the light falling on a chemical soaked film, it lands on a sensor. Think of the sensor as the camera’s eye, it “sees” the light and records the picture.
I don’t really want to use an eye as an analogy as your eye has a very clever brain behind it helping out. To simplify things your eye needs just the right amount of light to see. Too much light and you are blinded, time to put on your polarised sunglasses (more of this in another article I think), not enough light and it is too dark to see.
The “right” amount of light, is what’s known as the correct exposure. You are letting in just enought light to record a good photo.