A symmetrical plot is a plot line in which the number of conflicts equals the number of resolutions. Also, resolutions occur in the reverse order in which the conflicts arise. A Midsummer Night's Dream is indeed a symmetrical plot as described above.
There are about four different conflicts found in A Midsummer Night's Dream. The first can be seen as character vs. character, or character vs. society. Hermia is in opposition with her father's will for her to marry Demetrius, and as a result, she is being threatened with punishment by both her father and Duke Theseus, who represent society. The second conflict we learn about can be referred to as character vs. circumstances and also as character vs. character. We learn that Helena has become the victim of Demetrius's fickle nature. Helena's heart has been broken by both circumstances beyond her control and Demetrius himself. We can also see this as a character vs. character conflict because Helena wants Demetrius to love her again while Demetrius does not. The third conflict we see is again character vs. character. Oberon feels his wife has betrayed him and is jealous of the Indian boy she is doting upon. Fourth, Puck adds to the conflicts by mixing up the lovers, creating more character vs. character conflicts.
The plot is symmetrical because we pretty much see all of these conflicts resolve in nearly the opposing order they were presented. First, Oberon gives Puck further instructions for fixing the mess with the Athenian lovers. Oberon tells Puck to lead the lovers all over the woods until they drop from exhaustion, enabling Lysander to be re-enchanted and the women to fall asleep next to the men so that when they awake, all four will be properly paired, as we see in Oberon's lines:
And from each other look thou lead them thus,
Till o'er their brows death-counterfeiting sleep
With leaden legs and batty wings doth creep. (III.ii.380)
Next, Oberon decides that it is time to release his wife from the spell. Third, Egeus and Theseus discover the lovers in the woods, and Theseus decrees that Hermia shall marry Lysander, and that Demetrius shall marry Helena. Finally, the mechanicals perform their play and all of their illusions of grandeur finally meet reality in the form of being laughed at by their audience.