Nick Bottom is one of the central figures of the play. He is one of the workmen who get together in the woods at night to practice their version of the play, "Pyramus and Thisbe" with the hope of being chosen to perform it for Theseus and Hippolyta's wedding. He has a high opinion of himself and his abilities as an actor, though he isn't really a very good actor. We know this because he mispronounces words and he overacts which is evident when he performs the role of Pyramus before Theseus and the rest of the court. It is because he and the other workmen are not good performers that Puck gives Theseus the head of an ass. But it is with the changed Nick Bottom that Titania falls in love when she wakens. After the spell is removed from Titania and she no longer loves Bottom, that Bottom - who was never aware that he had the head of an ass - says upon waking himself that he has had a rare dream, a dream that cannot and should not be explained. Some things, he implies, are best not reasoned out. His speech here in Act 4, sc. 1, indicates that he embraces imagination and realizes that some things are best simply accepted.
Theseus is the Duke of Athens. It is his marriage to Hippolyta that brings many of the characters together. Theseus, in his speech at the beginning of Act 5, sc. 1, implies that he does not have the imagination or the belief in imagination that Bottom has. His words suggest that he thinks the four lovers made up their story about all having the same dream. He doesn't see the plausibility of such an occurrence. Theseus sees the imagination as something other people have and to understand them, sometimes a person has to accept that others use their imaginations.