惡露 is the blood that come out after giving birth. Just like your period. The blood is actually built inside the body to make room for a baby's development. For period, because no baby, so the blood comes out every month.
But for pregnant women, since there is a baby inside you, the blood will not come out because it will be used to nurture the baby. So no M come for pregant women. But after giving birth, since the baby is taken out from your body, the blood is no use anymore so it will comes out. Usually it is very heavy and much more than your normal period. That's why it is called 惡露. It can go on for 10 days. Only after 惡露 is cleared, the mother can take in nutritious Chinese medicine / chicken/ soup like that.
That's why people have to buy the pregnant women napkins, which is thicker than the normal ones.
For me, the blood is much more heavier for the single baby delivery. THe net underwear I prepared is not enough and I have to ask somebody to buy more for me when I was still in the hospital.