Colostrum - the "first milk" that the breasts produce as a precursor to breast milk. Breasts grow and develop throughout pregnancy and at around 16 weeks they start producing this rich, high-protein pre-milk, ready for when baby is born.
Unfortunately for me and my bras, the colostrum decided to start leaking as soon as my boobs started producing it!
But it doesn't happen to everyone. Some may not get their milk in until a day or two after the birth.