"Péter is not stupid, on the contrary! He is very smart."
Translation:Péter nem hülye, sőt! Nagyon is okos.
"is" can give more emphasis and /or possibly mean "on the contrary".
Another few examples:
- Nagyon is jó! - It really is good! (heh is-is :) )
- Nagyon is finom! - It really is delicious!
- El is olvastam! - I indeed really did read it! (emotional, eg. being proud of reading it)
An example with "on the contrary" meaning:
- Nem igaz! - It's not true!
- Nem is igaz! - On the contrary, it's not true! (expressing surprise or anger)
In this current sentence I say it means both. It could work without "is" as well, but using it gives more emphasis.