>This Pokemon and its allies' moves have their accuracy boosted to 1.1x.
It's as simple as that, every move Victini or the Pokemon on the field with it (only the Pokemon on its team, not the foe's Pokemon) use, get a x1.1 boost to its accuracy. This means the ability is only active while Victini is out on the field.
Hypnosis' accuracy is 60%. 60x1.1= 66% accurcay.