Highlights are simply pieces of hair that are lighter than your natural color. If your base color is brown, you can add highlights that are a lighter brown, or even have brown hair with blonde highlights.
For blondes, blonde highlights incorporate lighter blonde shades. It’s important to note that there are different types of highlighting techniques, too. Traditionally, foils are used to strategically isolate parts of the hair in various shapes and patterns, be that to create contrast that’s very bold, or very soft.
There’s also a newer technique, Balayage, where highlights are hand-painted onto smaller, more random pieces of hair, resulting in a more lived-in look.