What does it mean when my mechanic says the brake fluid is "contaminated"?

Most manufacturers recommend changing the brake fluid every two years irrespective of the kilometres travelled, due to brake‑fluid deterioration and contamination, which can cause brake failure. Brake‑fluid contamination can be caused by something as simple as dipping an oily finger in the brake fluid. Brake fluid is also hydroscopic (meaning it absorbs moisture), this can lower the fluid’s boiling point and cause brake system components to corrode internally, causing premature failure. Heat generated from the brakes can cause the water‑logged fluid to boil, which can cause sudden or unexpected brake failure. Brake fluid should be changed as recommended for your safety and that of other road users.