A simile is when a direct comparison is made between dissimilar things using "like" or "as". This literary technique is used to provide a more detailed and understandable description of the concerned elements/things.
The given quote is from Act I scene iii of William Shakespeare's "Romeo and Juliet". Spoken by the Nurse, the quote is from when the Nurse revealed to Lady Capulet about Juliet's childhood incident where she had fallen and hit her forehead. This resulted in a big bump on the forehead, which the Nurse admits is <em>"a bump as big as a young cockerel's stone."</em> The use of the comparison word "as" makes it a simile, where the bump is compared to a rooster's testicle.
