Although most suspect that Zibanejad will return to Djurgardens of the SEL this year, this contract gives the Senators the option of keeping him around if he impresses at training camp this year. Had the contract not been signed by July 15th, the Senators would have had to pay Djurgardens $100,000 and signed him by August 15th in order for him to be allowed to play in the NHL this season (thanks to The 6th Sens for the explanation).
Though sixth-overall picks generally aren't ready for the NHL the year they get drafted, Zibanejad actually does have a shot at making the team due to the Senators' mediocre centre depth. After Jason Spezza, the other centres are Peter Regin, Stephane Da Costa, Zack Smith, and Jesse Winchester, none of whom instill a lot of faith as second-line centres. Given Zibanejad's youth and inexperience, it seems unrealistic that he'll actually make the team and use the first year of that contract this year, but stranger things have happened.