What is meant by the happiness or misery mentioned in the Hadith, "Allah sends an angel who is commanded with four things: with his provision, with his lifespan, and whether he is wretched or happy"; are they in this world or in the Hereafter?

Misery is the opposite of happiness. Misery is a bad and evil state, and that which is repulsive to the nature of the one described by it. As for happiness, it is the good and benevolent states that are suitable for the one described by it. And the angel writes for the fetus whether it is miserable or happy, based on what is sealed for it.