they’re like a psychological theory about how we tend to group things together based on size, shape, color, proximity, etc. a few examples of how that applies in product design would be like how on your phone all the apps are the same size and shape and set up symmetrically and one after the other or how in a lot of logos it’s just a few white lines cut out of a colored background but to your eye you see the letter/name of the brand or like my personal favorite how the fedex logo uses negative space to have the logo double as an arrow shape etc