Explain what? He's saying that he wouldn't begin to sweat against Naruto because of his arrogance, and, also because he thinks he's more powerful than him.
He felt how great Naruto's power was. Tobi wants to be in control and he likes to think he's vastly superior to a kid like Naruto but for a moment he doubted himself and felt Naruto was too strong for him which didn't make sense to him