document.write insert string into the HTML page, before browser finishes reading the rest of HTML page.
document.write("<p>how are you?</p>");
This is a simple example of inserting text into a document. But this is not a good way.
document.write() is from 1996, when DOM didn't exist yet. It is a blocking construct. When browser encounters it, it stops everything and run the code.
document.write() is not used in modern web programing since about 2010.