Here are some examples:
* Lowercase letters with descenders: g, j, p, q, y
* Capital letters with descenders: J, Q, Y
Think of it like a "tail" that hangs down below the rest of the letter.
Descenders help to make cursive handwriting more legible and to give it its characteristic flowing appearance. They also help to distinguish between similar-looking letters, like "t" and "l" or "f" and "h".