What is em exactly relative to?

em is relative to font size of parent. HOWEVER, it is relative to its own element if it is applied to anything else other than font size. Below is example

section {
font-size: 18px
}
.abc {
font-size: 1.5em;
}
section {
font-size: 18px
}
.big-abc {
  font-size: 10px;
}
.abc {
padding: 1em; //here paddingwill be relative to .big-abc and not section tag
}
<section>
<h1 class = "abc big-abc"></h1>
</section>