How does rem differ from em in CSS

Question

In website sources, I have sometimes seen developers use the rem unit. Is it similar to em? I tried it to see what it actually does, but what is it relative to?

HTML

<div>Hello <p>World</p></div>

CSS

div {
    font-size: 1.4rem;
}

div p {
    font-size: 1.4rem;
}