67. Here signs mean those visible miracles which are presented as a proof of Prophethood. The disbelievers from among the Quraish were demanding such signs over and over again.
68. This is to warn the disbelievers, as if to say: It is the mercy of Allah that He is not showing such a sign so that you might mend your ways but in your folly you presume that such a sign cannot be presented. You should know that a sign is not being sent because its denial inevitably brings chastisement and the community is annihilated. You can learn from the past history that those communities which disbelieved in the open signs were utterly destroyed, such as the people of Thamud.
69. That is, miracles are not shown for fun. These are meant to warn the people that the Prophet is being helped by the Almighty and that they should realize the terrible consequences of disobedience.