يَوْمَ الْفَصْلِ (Day of Judgement): It is the Resurrection Day when  Allah (swt) will judge between the creation.

مِيقَاتًا (appointed): It is fixed at a definite time by Allah (swt).

If there was no judgement day, we would have asked for it to be, because although a human being might escape from justice in the world, he will never escape from the justice of Allah (swt). Therefore, beware of causing harm to anyone! He might not have any other means of complaining of your transgression towards him except his moaning in the darkness of the night which reaches the God of human beings. For the justice of the sky is extended to the transgressed. If a Day of Judgement does not exist, it would be a catastrophe as the justice should be observed in all the aspects not just in this worldly life.