It’s ‘bonus cash’ which means that they add $6 to your bid amount, if you bid $40 for a hotel night, they treat it as though it were a $46 bid. So using this coupon you’ll want to bid $6 less than you otherwise would.
The coupon can be used through December 31.
And remember that by going through Ebates you’ll earn 2% cash back on your Priceline purchases.
Update: Per Mike L in the comments, of course Big Crumbs offers 2.4% rebates on Priceline. Better than ebates.