In the first act, Egeus and Demetrius have the law on their side when it comes to determining to whom Egeus's daughter, Hermia, should be married. Theseus therefore feels bound by the law and the prosecuting parties to deny marriage between Lysander and Hermia. If Hermia does not comply with her father's wishes and marry Demetrius instead, she will be either put to death or forced to enter a convent.
However, by the time Theseus, Hippolyta, and Egeus happen upon the lovers in Act 4, one of the prosecuting party, Demetrius, has changed his mind and now loves Helena. This solves the problem in Theseus's mind so that Hermia doesn't have to die or become a nun. Egeus now has no man to wed his daughter save Lysander, which means that his claim to have Demetrius wed Hermia is void. In his mind in Act 4, the legal claim is forfeit. Therefore, Theseus decides to remedy the problem quickly by overriding Egeus's first claim to decide Hermia's husband by having all of the lovers married immediately with him and Hippolyta. Theseus thus declares:
"Fair lovers, you are fortunately met;
Of this discourse we more will hear anon—
Egeus, I will overbear your will,
For in the temple, by and by, with us
These couples shall eternally be knit" (IV.i.177-181).
I assume that you are talking about the part in Act IV, Scene 1,. In that scene, Theseus tells Egeus that he is going to override Egeus's decision about who his daughter will marry. Egeus had told Hermia she had to marry Demetrius and Theseus had backed him up. Now Theseus is contradicting his earlier statement.
Back in Act I, Scene 1, Theseus told Hermia that she had to take her father's word like the word of God. She had to do what he said or be executed (or enter a convent). But now he is going to overrule Egeus and order that Hermia and Lysander and Demetrius and Helena all be married at the same time he marries Hippolyta.