>It makes your red blood cells far more resistant to being infected by the disease-causing agent, the plasmodia

It's because one of the life stages of malaria requires for the parasite to mature inside red blood cells (RBCs), before the cell bursts and all the mature parasites are released into the blood stream.

With sickle cell disease, the RBCs are more fragile and burst more easily. That means the RBCs are bursting and releasing the parasites into the blood before a lot of them are fully mature, which hurts the progression of the disease.

In the presence of two copies of the gene (one from each parent), that trait is even more pronounced and RBCs are so fragile that they're constantly bursting for no reason, which causes anemia and all sorts of body-wide problems. Day to date events like stress, high temperature or dehydration can cause them to burst en masse and cause pain attacks.

Also the name is because the RBCs are literally shaped like a sickle instead of being kind of round.