Romney is saying that he's been limited on how much he can spend running against Obama because he's still in the primary season. The Romney campaign spent all their primary dollars advertising for the primary. After the convention, they can switch to their general election fund. Obama, on the other hand, has been able to spend freely on attack ads against Romney because he didn't have to deal with a primary battle.
So in other words, Romney and Obama are polling neck and neck currently, but that's with Obama spending freely to attack Romney while Romney has one hand tied behind his back.
The GOP convention is August 27 -30. After that, I expect things to start getting really interesting.