Legible describes readable print or handwriting. If someone tells you that your writing looks like "chicken scratch," it might not be legible, except to other chickens.
Legible goes back to the Latin word legibilis, meaning "that can be read." If you can read someone's handwriting, it is legible. The person might not have perfect penmanship, but if you decipher the letters, the writing is legible. People's signatures are especially notorious for not being legible — that's why we often must print or type our names below.