We all want the best from our efforts to produce stunning astrophotography.
The road to excellence is paved with many frames.
Stacking, or integration, is the combining of multiple frames to get the most signal and the least noise.
Although you could possibly take a single one-hour long exposure, there are several reasons why you wouldn't. Instead, you would replace that with 6x600 seconds to achieve the same overall exposure.
One of the reasons is that as you build time, you build inherent noise from the sensor. The longer the sensor is activated, the more electrons are swimming around, interfering with collecting the photons.
The other reason is that long exposures leave you open to stray artifacts like planes, satellites, and cosmic ray hits.
The shorter exposures minimize these problems.
But wait! There's more. A short exposure, in any type of photography, lets in less light. Your image (signal) is dimmer.
We can make up for that by stacking the frames. Each frame you add gets added to the light obtained for the image. This brightens the object of interest, the signal. Fortunately, each image's noise doesn't add up quite so quickly because the noise of the individually exposed pixels is random in nature. Imagine building with interlocking plastic blocks on a grid. Place one wherever the photons go, and you will see a scattering of odd noise while the signal grows larger and larger until a recognizable and sizable difference occurs.
Integration is used on a set of subframes (same filter). The software stacks and adds the frames together and statistically removes pixels that don't line up on other frames. This results in a cleaner image.
Finally, you combine the integrated set to achieve a color image.
You will hear talk of integration time, including the use of multiple observing sessions, such as One-Click observations taken at different times. Additionally, you can collaborate with other observers and borrow their data to add to your own.
This "deep" integration resulting in hours of exposure time gives you more and more signal. More signal means more detail and color.