Prismjs is a lightweight, extensible syntax highlighter primarily used for enhancing the appearance of code snippets in web pages. It supports a wide array of programming languages and is renowned for its ease of use, modularity, and ability to be customized. Prismjs can be tailored through plugins and themes, allowing developers to extend its functionality and appearance according to their needs. It is particularly popular for its performance and flexibility, enabling developers to highlight code without overburdening the web page.