What is a pseudo-element selector?

In CSS, a pseudo-element selector applies styles to parts of your document content in scenarios where there isn't a specific HTML element to select. For example, rather than putting the first letter of each paragraph in its own element, you can style them all with p::first-letter.

A CSS pseudo-element is a keyword added to a selector that lets you style a specific part of the selected element(s). For example, ::first-line can be used to change the font of the first line of a paragraph.

/* The first line of every <p> element. */
p::first-line {
  color: blue;
  text-transform: uppercase;
}

Note: In contrast to pseudo-elements, pseudo-classes can be used to style an element based on its state.

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/Pseudo-elements