Just like it sounds, a fetal blood transfusion is an emergency medical procedure whereby blood is transfused to a fetus still in utero.
In this delicate procedure, a long needle is inserted into the mother's abdomen and guided via ultrasound to either the umbilical vein or a vein in the fetus's own abdomen. Because the procedure is so delicate, the mother must be sedated to prevent movement, and often the fetus is sedated as well.
The main reason to undertake a fetal blood transfusion is fetal anemia, whereby the fetus has a potentially fatal dearth of red blood cells. This condition most typically arises as the result of either a viral infection or the mother developing a sensitivity to Rh factor present in the fetus's blood, leading the mother's own immune system to start destroying the fetus's red blood cells.
As mentioned, a fetal blood transfusion is quite a difficult procedure, and is generally performed only by the most skillful and experienced doctors. Even so, the procedure results in the death of the fetus in roughly 10 percent of attempts, and thus is only attempted in dire emergencies.
I have sometimes heard this procedure referred to as an "intrauterine fetal blood transfusion," but this seems pretty redundant to me as once a baby is out of the womb, for whatever reason, it's no longer a fetus, and the procedure becomes just a regular old blood transfusion.