Css stands for cascading style sheet.
HTML is not able to change html elements or tags look and feel or style, it means it is not able to change elements color, background color, font size, height, width, border and so on. Here CSS is used to change html elements style so, that it makes html elements more presentable and friendly. It is also used for responsive web design to display HTML elements according to different screen sizes.
CSS files are saved with .css extension.