content - CSS | MDN

The content CSS property replaces content with a generated value. It can be used to define what is rendered inside an element or pseudo-element. For elements, the content property specifies whether the element renders normally (normal or none) or is replaced with an image (and associated "alt" text). For pseudo-elements and margin boxes, content defines the content as images, text, both, or none, which determines whether the element renders at all.