Proximity is about how near or far objects are from one another.
Our brains make a lot of assumptions for us based on where things are located in relation to one another. People assume that there is a relationship between things that are together, even if there isn’t. We just assume that things that are close together, belong together. Think of salt and pepper shakers, or two friends meeting in the park. When we see a couple with their heads close together, deep in conversation, we know that they are ‘a couple’ and not just friends. When you are designing documents or slides, always put related information together so that people can easily see that it’s related. It’s equally important to use lots of white space to separate information that’s not related.
This seems really obvious, but its a design principle that is often neglected.