6/9 and 6/12 means you have poor vision in both your eyes and you are rightly using glasses. As mentioned by Dr Seema it is a way to measure eyesight, and the technique used is known as Snellen's chart, just as most other things like milk, sugar, gas etc are measured

Hi.. The vision is checked in terms of visual acuity....which in Snellens chart measured as 6/6, 6/9, 6/12..n so forth, 6/6 being normal.for distance The Eye power or spectacle number is measured in dioptres as in -1.0, -2.0 etc... If it helps .....e.g. that with -2,0 D power, if your vision is 6/6 , thats good...If not improving that much either the glass power is not correct or there is a reason for vision not improving to normal...