What does an empty value for the CSS property content do?

Question

nav ul:after { 
    content:""; 
    clear:both; 
    display:block;
}

What does the "" value mean on the content property?

Answer 1

It's just a clearfix, which means, if your ul elements have floated li, than they will be self cleared, it just behaves similar to overflow: hidden; but unlike overflow: hidden; it wont hide the overflowing content.

Demo (Without clearfix)

Demo 2 (With clearfix)

And why you need to clear and what you need to clear, for that, you can refer my answer here

---

As you edited your question, content property is used with :before or :after pseudo to embed virtual content on the page, so using content: ""; means just blank, you are not embedding any character in your page.