That is Salam to you. Its predicate is ‘upon them’ but another explanation is that it could be the object of the verb. That is, Salam on the people of paradise from the Merciful Lord.
The apparent meaning of this Holy verse is that the Salam is without any intermediary, directly from the Lord of the worlds. Salam for the Faithful is usually through mediatories. At the moment of death, it is through the angel of death and his assistants.