Narrowing of the aortic valve in the heart.
The aortic valve controls blood flow from the left ventricle to the aorta and the rest of the body. It opens when the ventricle contracts and closes when the ventricle relaxes.
Aortic stenosis means that the aortic valve has become narrowed. The result is that the left ventricle must squeeze harder to get a sufficient amount of blood through the aortic valve with each beat. (Imagine trying to push the same amount of water through a small syringe needle...