A README file (often written in all capital letters to draw attention) makes data more understandable and reusable, whether by your future self or others. In data sharing, a README is typically a plain text or markdown file that briefly describes the organization of everything else included, like the table of contents in a book. A good README's strength lies in describing relationships among files, and can be the best place to include certain details about reproducing scientific analysis.
Here is an example of a README from a Figshare submission:
Setting up a README early in a project can help capture key information about datasets as they are being created, especially if many collaborators are involved in the collection and analysis processes.