Whenever you take a photo, the image sensor is exposed to light for some amount of time. The duration of this time period where the sensor is capturing light is called "exposure time" or "shutter speed." By increasing this duration, the sensor receives more light, which is useful for taking photos of dim objects (for example, a 2 second exposure gets twice the light as a 1 second exposure). However, this has a tradeoff - if the camera or object moves before the exposure finishes, then there will be blur in the image. This is why photos taken under low light tend to be blurrier, since there is more of a chance that camera will shake in your hands or the subject will move.

When you take photos in daytime, you usually use a shutter speed of from 1/8000th of a second to 1/30th of a second. However, at night photographing dim objects, these kinds of shutter speeds will not give you any reasonable signal - more or less just a black frame. As such, you take exposures of 1 second or more to gather as much light as possible without incurring blur from the sky rotating (tracking mounts help with this).

However, for dim objects like galaxies, even 1 second exposures don't capture enough signal compared to the overall noise level of the camera. As such, we can take many 1 second exposures and stack them together by aligning the frames (done by aligning the stars in the image together) and then adding the pixel values. This results in an increased signal to noise ratio, allowing us to make a more detailed image once properly edited.