The source code is the programming behind any webpage, or software. In case of websites, this code can be viewed by anyone using various tools, even your web browser itself. However, this is a little difficult on Android and iOS phones.
The page source is also very useful when you are doing search engine optimization, or SEO. A search engine only sees the website's source code. Even if your web page looks perfectly fine to you, it may not match the standards required by a search engine crawler. Make it a habit to look through the code, especially the important header tags, such as title, and description.
This tool allows you to view the formatted version of the source code of any website.