The load index is a code, not a weight. 91 means 615 kg per tire, 94 means 670 kg, 100 means 800 kg. Multiply by four for the axle set, and remember that figure is for a correctly inflated tire.
The speed rating is a letter: T is 118 mph, H is 130, V is 149, W is 168, Y is 186. It describes the speed the tire can sustain safely when properly inflated and loaded, not a target.
Fitting a lower rating than your manufacturer specifies can affect handling, load capacity and in some countries your insurance. Going higher is generally safe but often means a firmer ride and a higher price.
If you tow, carry heavy loads or run a full car regularly, pay particular attention to the load index rather than the speed rating.