To keep loose papers in order, they are frequently put in large, folding pieces of stiffer paper called folders. These are commonly of a beige or “manila” color (due to being more durable, unbleached paper originally) and often have a raised tab on one side and corresponding cutout on the other, making a space to write a label.
In computer environments mimicking a physical desktop, documents are grouped together in digital folders recognizable by their shape and sometimes a manila or other off-white color. The documents within may even peek out as if the folder is opened.
Interestingly, sometimes the folder is rendered as if it is standing up and opened on its side, like a book. This is dangerous for a real folder, but makes sense on a screen for showing documents in left-to-right order within.