Pretty-printing is the reproduction of input or output content in its “most concise” form. For instance, you may know that lists are a series of nested cells that are null terminated – like this: [1 [2 [3 [4 ~]]]]. That aforementioned list is pretty-printed (based on a variety of rules which you will learn) as ~[1 2 3 4]. Pretty-printing is intended to make output more human legible and also more aesthetically pleasing.