You hear this advice everywhere you look when it comes to astro imaging. It’s pretty much true. Dithering is an event that occurs between exposures, where the mount is moved a little bit in a random way to point a few pixels away from where it just was. This is used to combat noise.
Things like dark noise and noisy flats can contribute pattern noise when calibrated into the light images. This noise can be accentuated by the stacking process. For example, consider the following calibrated integration that did not use dithering:
You can see that pattern noise is streaked across the image due to slight right ascension shifts throughout the imaging session. If the tracking had been perfect, the noise would be even more pronounced as it was integrated.
With dithering, the noise is much more consistent and random. This was produced with several hundred shorter exposures that were dithered about 7 arcseconds every third frame. The resulting noise is much easier to reduce with a denoising algorithm in post processing..