mm is a measurement of sunglasses size. It is short for millimetre. In fact, apart from sunglasses, eyeglasses are also measured by mm. And the sunglasses size is very important for people who bought sunglasses online since they have no idea if the sunglasses fit for their face. But if you bought them from eyeglasses shop, you can have a chance to try them, that is ti say the sunglasses size is not too much for them. Anyway, if you buy online sunglasses, you shall know what sunglasses size you need so as to get suitable sunglasses.
Well, I think this is very easy to understand. One of them refers to the filter ring size while the other one refers to its focal length. I know they may sound weird to you but actually this is really very easy to understand. The former one means the distance between the two arms of your sunglasses but the other one means the size between your two size. Just about this, I wish this could help you.