A hectare is 10,000 square meters: a square of one hundred meters on each side. It is the unit in which the rural area is measured and in which land is valued throughout the country.
One millimeter of rain on one hectare is ten thousand liters of water, which helps to understand why rain is measured in millimeters and not liters.
The surface area shown by the site is calculated on the polygon: that of the plot if it was chosen from the cadastre, that of the drawing if it was drawn by hand. It may not match exactly the one in the title.