What is a summary of Bottom's dream in "A Midsummer Night's Dream"?
I think your question refers to Bottom's soliloquy in Act IV Scene 1 when he wakes up after the four lovers have been discovered in the woods by Theseus and Hippolyta. He has had his head of an ass removed, and we see him reflecting on what has happened to him.
A key theme throughout the play and one picked up throughout this scene, both with Bottom and with the lovers, is the relationship between reality and illusion and how we can discern the difference between them. Bottom, reflecting on what has passed, realised that his dream was "past the wit of man, to say, what dream it was." His experience was so fantastical that words don't have the power to communicate it. Indeed, Bottom continues, men are asses if they try to explain the dream. Not every event can be explained rationally, and some things are better off remaining in the realm of imagination and fantasy. Human senses are incapable of capturing such realities ("The eye of man hath not heard, the ear of man hath not seen, man's hand is not able to taste, his tongue to conceive, nor his heart to report, what my dream was"). Only literature has the power to capture such dreams, which is why Bottom will have Quince write a ballad of his dream.
The reason for this ballad being called "Bottom's Dream" is because it has "no bottom", perhaps reflecting that works of literature have no bottom - that is, they cannot be quantified, measured or understood solely through the means of logic. Our impressions of a play or novel or poem do not stay static - they change, and there are as many different interpretations of a work of literature as there are people. Shakespeare here therefore is perhaps reflecting on another key theme - that of the conflict between reason and imagination.
When Bottom falls asleep in Act IV, Scene I, it is to the gentle tones of Titania telling him "O, how I love thee! How I dote on thee!" Just before he wakes, we hear Demetrius say, almost as a cue to Bottom, "And by the way [as we are going] let us recount our dreams."
It is at this point that Bottom wakes up and declares that he has had "a most rare vision." However, he is so troubled and confused by the strange content of his dream that he feels "man is but an ass, if he go about to expound this dream" (only an idiot would attempt to explain the meaning of this dream.) Bottom hesitates in his recollection--"Methought I was--" while the audience appreciates the dramatic irony, knowing (or thinking they know) exactly what Bottom had seen of himself. However, Bottom never actually speaks anything of what his dream contained: he says he will have Peter Quince make a ballad of the dream, to be called "Bottom's Dream, because it hath no bottom." That is, the intricacies of his dream are so endless that "only an ass" could think himself capable of ever fully understanding what it could possibly mean.
This comment is, of course, also a reference to Demetrius' prior comment that they should recount their dreams to one another: Bottom suggests that anyone thinking himself capable of expounding and understanding dreams is surely mistaken at best, and at worst, a fool.