There’s a Crack in the skin of the Universe on Trenzalore. The Time Lords use this Crack to push a message into the Universe (“Doctor Who?”), waiting for a response (the Doctor’s name) which will lead them home.
If the Doctor says his name, the Time Lords come back and the Time War recommences. If he leaves, the planet will be slaughtered. So he stays but doesn’t say his name. The Church of the Papal Mainframe becomes the Church of the Silence - devoted to keeping the planet protected, so that the Doctor’s hand is never forced.
But the Kovarian Faction takes a more extreme view. They decide to alter history so that the Doctor can never reach Trenzalore at all. They travel back and their first plan is to destroy the TARDIS - unwittingly creating the Cracks in the first place with the resulting explosion.
The Doctor calls this the Destiny Trap. Kovarian travelled back to prevent the Doctor reaching the Crack, but the Crack only exists because Kovarian travelled back.
The most succinct explanation of the crack paradox I’ve heard thus far.