Although I am not an optometrist, I will try to make my explanation of this as simple as I can. First, lets go over the plus/minus before the number. Plus
means the glasses magnify, minus means they minimize. The numbers are called "sphere powers", which indicate the amount of focal length your contact lens will correct for. The difference from the -3.50 to -3.00 means the the distance for you to be able to see things clearly has changed from 1/3.5 meters to 1/3 meter. Anything farther than 1/3 of a meter would be blurry to you.
If you have more questions, I suggest you speak with your optometrist.