An EM (Emphemeral Unit) is a unit of measurement relative to the font size of its parent. As such, when the parent font size changes, the size of the child font changes automatically.
For example:
If the parent font size is 16 px and the child font is set to 1.5 EM, the child font is 16 x 1.5, or 24 pixels.
If the parent font is changed to 20px, then the child font will automatically be 20 x 1.5, or 30 pixels.