If you bet on 0 or 00 on an American wheel, and your bet hits, you'll get a payout of 35 to 1. If you place a "row" bet (betting on both 0 and 00), and your bet hits, you'll get a payout of 17 to 1.
The 0 (single zero) and the 00 (double zero) are the green slots on the roulette wheel. These pockets could receive a 35:1 payout if hit straight up. If there's a split, the payout could be 17:1. This type of bet is somewhat risky, but the payouts could be worth it.