Among werewolves, the Veil is poetic term for the false assumption that the supernatural does not exist in the World of Darkness, which the Delirium reinforces. The Veil was formally instituted through the Concord of nine thousand years ago, which ended the Impergium.
The Litany states clearly that "The Veil Shall Not Be Lifted".
The Veil is similar to the concept of the Masquerade among Kindred and Consensus among the Mages. Delirium helps in keeping the Veil in place. Harming or rending the Veil means a great loss of honor among the Garou, while repairing the Veil means the opposite.