Your heart is made up of four chambers: the left and right atria and the left and right ventricles. The left atrial appendage, or LAA, is a pocket-like section of one of these chambers, the left atrium.
As your heart beats, the chambers contract and expand to push blood through your heart and out through arteries to the rest of your body. If you suffer from atrial fibrillation, a disorder that can prevent your left atrium from contracting fully, blood can pool in the LAA and form a clot. The blood clot can then break free and travel through your arteries, potentially reaching your brain and causing a stroke.