"Hair layers before and after" refers to the comparison of a hairstyle before and after adding layers, which are sections of hair cut to different lengths. For instance, one might have long, one-length hair "before" and shorter, layered hair "after" a haircut.
Styling hair with layers is relevant because it adds volume, texture, and movement. It also allows for more versatile styling options, such as braids, ponytails, and updos. Historically, layered hairstyles gained popularity in the 1970s with the rise of shag haircuts and have remained a staple ever since.