7.15 is referring to the date on which the final movie will be released in most cinemas, 15th July. The horcrux thing was explained in Half-Blood Prince, but they may not have covered it as well in the movie.
The book gives you all the details. All you need to know is this the horcruxes are what prevents Voldemort from dying. To be able to destroy Voldemort, they must first destroy all the horcruxes. Dumbledore has speculated that there Voldemort split his soul into seven pieces, meaning that he created six horcruxes (one piece remains in his body). By the end of Deathly Hallows: Part 1, they have destroyed three – the diary, the ring and the locket.