The exposure time is the time duration for which the image sensor is exposed to light.

Since photography is about capturing one still moment it differs from the eye when it comes to light capture. The eye, similar to a movie camera, captures movement and is a continuous process which is always gathering light.

The camera acts on a medium, film/sensor, which captures one scene. This means that the shutter allows light to fall on the sensor/film for one instant and then closes the light away. Much like opening and closing a door really fast.

when you point the camera at an object, the camera chooses the right shutter speed and aperture to gather the perfect amount of light to make a picture.

This gathering of the perfect amount of light is called exposure (you are exposing the film/sensor to light for a period of time).