`rem` (Root EM) is a unit of measurement in CSS that stands for "root em". It is similar to `em`, but `rem` is relative to the root element (i.e. the html element) of the document, instead of the parent element.

`rem` is used to define font sizes, margins, padding, and other properties of an element. One `rem` is equal to the font size of the root element, which is usually set to 16px. You can then specify other font sizes or other values in terms of `rem` to create a consistent scaling of your layout.

For example, if you set the font size of the root element to 16px, you can set the font size of a heading to 2rem, which would equal 32px. This allows you to have a consistent scaling of your layout, as all other values that are defined in `rem` will be relative to the root font size.

Using rem can make it easier to create a responsive design, as you can adjust the root font size for different screen sizes to adjust the overall scaling of your layout.