Atrial fibrillation, also called A-fib, can cause a stroke as a result of the irregular beating of the heart's upper chambers, which sometimes results in blood forming clots in the heart. (A normally beating heart moves all blood along so efficiently that clots can't form.) Once a clot has formed, the movement of the blood in the heart can sweep that clot out of the heart and into the brain. Strokes caused by such clots can cause serious disability or even death.
To prevent such strokes, many people take a blood thinner, such as warfarin, dabigatran, or aspirin.
Although blood thinners can contribute to serious bleeding, especially after a fall, in most people the decrease in stroke risk substantially outweighs the risk of bleeding.