Glasses are worn by a large part of the population, so the term is relatively well known. Lenses are an essential part of all prescription glasses, and their "power" is indicated by a unit of optical power called a Diopter. Lenses are divided into so-called converging and diverging lenses, converging lenses are used for close-up glasses (farsightedness), a plus sign is placed before the number indicating the dioptric size (for example + 3D). Diverging lenses are used in glasses for distance (myopia), they use a minus sign.
Technically, diopter is defined as the inverse of the focal length of a lens or convex mirror. It is not a unit of the SI system, therefore diopter does not have a designated symbol, most often "D" or "dpt" is used.
If we have an eye defect requiring wearing glasses, the eye doctor will measure our eyes and, based on this measurement, the optician will insert the correct lenses into our glasses. In the case of a combined defect, when we need both reading glasses and distance glasses, we can choose so-called bifocal lenses composed of two lenses ground close together, or better, the novelty of recent years, the so-called multifocal lenses, which combine the converging and diverging lense in one unit, where the transition from one type of lens to another is smooth and less disruptive. Whether the glasses are strong or weak is easy to tell and we don't even have to look through them, we can tell by the thickness of the lenses, because with increasing diopters, the thickness of the lens increases.