These systems are used to stack night time wide field scenes that include stars and landscape. Most often Milky Way images. Typically multiple images are taken using a tracker and then combined to reduce noise and capture more detail.
Sequator
Sequator is free Windows-based software. It is a tiny and user-friendly tool that compensates for the lack of exotic gear and that provides a quick way to stack non-blur star images in short-time exposures.
Its role is to control the exposure and average random noises. The program comes with a simple interface that is easy to navigate regardless of your experience with similar tools. While on the left side, you can add the base, noise, and clear images, in the right panel you can preview your work in real time.
Starry Landscape Stacker
Starry Landscape Stacker is a paid for Mac app for making images of the night sky with stars as points and low-noise. It uses “stacking” or “image averaging” to combine a group of images that were captured in rapid succession with identical exposure settings and the camera in a fixed position. The result is an image with stars as points and much less noise than you could achieve with a single exposure.