As an example, if you have a 15% probability for tornadoes, this means you have a 15% chance of a tornado occurring within 25 miles of your location. This may seem like a low number, but a tornado is very uncommon at any one location. Normally, your chances of getting hit by a tornado or other severe weather are small, purely based on statistical average. Let's say you have a 1% statistical (climatology) history of tornadoes within 25 miles on this day, which still is large. Having a 15% probability means 15 times the normal odds of a tornado nearby, meaning it should be taken seriously. The probabilities for severe thunderstorm wind and hail also have the same meaning as they do for tornadoes, but typically will be higher numbers than for tornadoes, since they are much more common.
Sometimes, a black hatched area will be overlaid with the severe probabilities. Black hatching means a 10% or higher probability for significant severe events within 25 miles of any point. "Significant" is defined as: tornadoes rated EF2 or greater, thunderstorm wind gusts of hurricane force (74 mph) or higher, or hail 2 inches or larger in diameter.