Plus sizing your wheel & tire
combination was designed to enhance vehicle performance and looks by allowing fitment of larger diameter rims and lower profile tires. The theory is that while making these changes, you keep the overall tire diameter within 3% of the original equipment tires. This is important because larger variances can cause problems with transmission shift points which can decrease fuel mileage. It can also confuse braking system computers which can even lead to brake failure.
Here's the rule of thumb for "plus sizing":
Increase section width by 10mm
Decrease aspect ratio by 10 points
Increase rim diameter by 1 inch
Increase section width by 20mm
Decrease aspect ratio by 20 points
Increase rim diameter by 2 inches.
This is not exact, but it will usually get you in the right ballpark. We always recommend consulting with the people you are purchasing the wheels and tires from to ensure fitment.