Going into detail on what this term means and how it relates to health guidelines. This article may be incomplete.
Thickening and calcification of the aortic valve leaflets causes restricted and limited mobility of those leaflets, causing a relative obstruction of blood flow.
Similar to atherosclerosis, it begins with lipid deposition in the leaflet tissue which then causes inflammation - which causes calcification and fibrosis (scar tissue). Calcification begets calcification. The more calcification and fibrosis that accumulates the stiffer and crunchier the leaflets become, which doesn't allow them to open as easily or fully, nor seal properly.
Think of it like your thumb over a garden hose. As you narrow the orifice, the velocity goes up and the "jet" is more intense. The heart has to work harder to pump the volume of blood out.
When severe, it prevents the heart from pumping blood across the valve into the aorta which delivers it to the body. This may result in syncope (passing out), lack of blood flow to the coronaries (ischemia) manifesting as angina; and ultimately heart failure (pump failure) and death.
It can also manifest more as a leaky valve (aortic regurgitation) due to not sealing properly.