Minification is the process of removing unnecessary or redundant data without affecting how a resource is processed by the browser. Minification can include the removing of code comments, white space, unused code, as well as the shortening of variable and function names. Minification is used to improve web performance by reducing file size. It is generally an automated step occurring at build time.
As minification makes code less human legible, developer tools have 'prettification' features that can add white space back in to make it a bit more legible.