IOR stands for ‘index of refraction’ which refers to just how much refraction the lenses in a pair of glasses provides. The higher the IOR then, the more refraction the glasses are performing and the more ‘corrective’ they are (the magnification qualities come from the refraction, just as they do in the lenses of the eyes themselves).
The problem previously was that the greater IOR required thicker glasses in order to provide the space necessary for this. This resulted in glasses that looked very thick and were of course not particularly fashionable to look at (they would require thicker, and therefore darker frames at the same time by extension). Where these glasses were made form actual glass, this would also be heavy and make the glasses more tiring to wear and more likely to fall off and as they could shatter this would also of course be more dangerous.
As such, as technology progressed, corrective glasses began being made from plastic which was lighter, more durable and much safer to wear. As technology has progressed further this has resulted in that plastic being thinner meaning that high index plastic glasses can now have a high refraction rate while being thin and light. Thus you can have highly corrective glasses and still look stylish with thin glasses anywhere up to IOR 1.74.