PhantomJS is one of the first major browser automation toolkits. It's a headless browser manager that's often used to web scrape using real web browsers to avoid blocking and rendering javascript pages.
Today, Phantomjs is superseded by a new set of tools that are more reliable, faster and easier to work with:
Note that modern browser automation tools use CDP to communicate with the browser. Because of this, today there are many different tools like PhantomJS.
For more on web scraping using headless web browsers see our complete introduction which covers everything you need to know about this subject