CSS, or cascading style sheet, is the document needed to define what the content of a webpage or app will look like. For example, the fonts and colors of a webpage are found in the CSS document. CSS is not a language that can be used to program a web page or app by itself. Rather, it tells the computer how it should display the data found in other files.
An HTML5 file will tell the browser where the header is and what text should be displayed in that space. A CSS file is what tells the browser to show that text in black, Arial, 11-point font. CSS is usually the second language that a programmer learns after HTML5.
Different browsers will display CSS differently, which makes it important to understand how content will appear different from one user to the next and to explain to different browsers what to do with the content presented.
A professional mobile app development company may be able to provide more information regarding CSS and other coding languages.