Wheel installer saids i need hub o rings in order to install after market rims?

Went to install after market rims (American Racing Haze (Series AR363) Gloss Black With Machined Lip - 17 X 7.5 Inch Wheel) on my car (2008 Honda Accord EX-L Sedan) with 225/50R17 Nitto NT450 Tires. I went online to see the specs of the wheels and they say plus one...does anyone know what this means? Also, where can i buy hub o rings in order to install these wheels? Please help!