A revel is a celebration. Theseus has planned this celebration because of his marriage to Hippolyta. It's an important part of act I because it's the reason Bottom and the other workers decide to rehearse for a play.
Theseus orders a revel to celebrate his marriage to Hippolyta. He tells Hippolyta in Act I, Scene 1
"Hippolyta, I woo'd thee with my sword,
And won thy love doing thee injuries;
But I will wed thee in another key,
With pomp, with triumph, and with revelling."