What is significance of em tag?

Answer 1

The <em> tag places emphasis on a word or phrase. Whatever you place inside the <em> tags will be rendered as italicized text by the browser

Answer 2

The <em> tag represents stress emphasis of its contents, while the <i>tag represents text that is set off from the normal prose, such as the name of a movie or book, a foreign word, or when the text refers to the definition of a word instead of representing its semantic meaning.