It means the surface is 195mm wide, the sidewall height is 65% of the width (195x0.65=127mm) and they fit a 15 inch wheel, 195-65-15. These give all the important dimensions of the tyres and shouldn't be changed. Doing so will give you a bigger or smaller tyre which could confuse your car's computers, causing ESP and ABS faults as well as affecting the accuracy of your speedometer.
91 is a load rating, you can look this up on a chart but 91 equates to a rating of 615 kilos. This is the maximum designed load to be carried by the tyre.
V is a speed rating, again you can look this up but V equates to 149MPH. This means the tyre is designed to cope with speed of up to 149MPH.
The load rating and speed rating are less critical if you stick to speed limits and bear in mind the weight of your car and its contents.