In Act I, Scene III, the nurse tells two stories about Juliet when she was a child. The first is a story about how Juliet was weaned from breastfeeding when she was three years old. The nurse put some "wormwood" (a bitter shrub) on her breast so that the taste would discourage Juliet from feeding. The nurse's plan worked: Juliet got "tetchy" (irritated) with the breast and never fed again.
In the second story, the nurse relates how Juliet fell and "broke her brow" (cut her forehead) while running around as a toddler. To comfort Juliet, the nurse's husband picked her up and said
"Thou wilt fall backward when thou has more wit."
This is a sexual reference: Juliet will have sex ("fall backward") when she is older and wiser. To this observation, Juliet stopped crying and answered yes, a fact which amused the nurse's husband.
These stories are important because they illustrate the Nurse's character: she is a source of bawdy comic relief as well as a significant and loving force in Juliet's life. Both stories also feature some element of hardship for Juliet. As such, in some sense they foreshadow the tragic events which will happen later in the play.
Both of the nurse's stories appear in Act 1 Scene 3 in the Nurse's monologue. The nurse and Lady Capulet are arguing about Juliet's age. The Nurse insists that she's 14--or will be at Lammas-tide (a festival on August first at which bread is made from the first harvest of corn and is blessed). Lady Capulet insists Juliet is not fourteen. The nurse recalls an earthquake eleven years before just before Lammas-eve, when Juliet was 3. She remembers because she was weaning Juliet.
She weaned Juliet (of all the days she could have picked) on the day of the earthquake. She did it by laying "wormwood to [her] dug"; that is, she smeared wormwood (a bitter plant) on her nipples so the baby Juliet would naturally reject them. Juliet tasted it and pitched a fit (she was "tetchy" and did "fall out with the dug," or reject it). Because of the earthquake, the dove-house she was leaning against at time started to shake, so the Nurse picked up Juliet and left.
At that time, the Nurse remembers, Juliet could already stand without help. She could have run everywhere. Just the day before, as a matter of fact, she'd fallen forward and "broke her brow." The Nurse's husband (now deceased) picked her up and said, "It's smarter to fall backwards," to which Juliet (Jule here)--who was already verbal--said "Ay."