What is the rift in the spirit room at the beginning of A Midsummer Night's Dream?

In A Midsummer Night's Dream, Oberon orders Puck, his chief fairy counselor, to drop the juice of a purple flower on Titania's eyelids as she sleeps. This will make her fall in love with the first creature she sees upon awakening. Oberon has Puck also drop the flower juice in the young lovers' eyes, restoring the lovers to their rightful partners. A bit of a mix-up occurs but all turns out well in the end, and everyone weds his or her apportioned love.