Ah, this is the most wonderful scene and sets the tone for the whole play. It is an enactment, in a sedate, courtly way, of one of the primary dances between the sexes. Theseus is filled with desire for his bride-to-be, hot and eager. Hippolyta, the Queen of the Amazons, whom Theseus captured in battle, then wooed for his wife, reacts to his amorous expressions with a seductive reticence: he say that the four days will be hard for him to bear. She says that they will pass easily, each day, one after the other, and before you know it, it will be the night of their nuptials. Theseus responds to this by ordering manifold diversions and entertainments as a way for him to distract himself from his impatient desire, and, of course, to honor his bride-to-be and give pleasure to the populace.
In a nutshell, the four days will pass for Theseus with uncomfortable impatience and seem like forever, while Hippolyta expects to experience them with perfect ease and as no time at all.