A sideburn is a length of hair that grows from the temple and down to the jawline.
The word "sideburn" is commonly used to describe facial hair, specifically this distinctive style. You might see it on old-timey photographs or on someone who's trying to emulate the look of a 19th-century gent. It's not just for historical reenactors though – some people like the way sideburns frame their face and add a bit of personality to their appearance. If you're talking about hairstyles with friends, "sideburn" is probably what they mean when they say that style.
Facial hair reaching from the top of the head down the side of the face to the side of the chin.
