The quick answer here is that they agree to run away from Athens and go meet in the forest.
In Act I, Scene 1, Theseus has just told them that they cannot get married. Hermia has to marry Demetrius because that is who her father has chosen for her. But Hermia is in love with Lysander and so the two of them are looking for some way out of this dilemma.
Lysander tells Hermia that he has a rich aunt. She lives a long way from Athens. The two of them will go, meet in the woods, and then head off to the aunt's house. The idea is that they will get married there.
In Act I Sc.1 The Duke of Athens Theseus warns Hermia that she must marry Demetrius the man chosen by her father Egeus or else she must die or become a nun:
Later, when they are left alone Lysander suggests to Hermia that they elope to a faraway place where the Athenian law cannot harm them. Lysander tells her that he has a rich widowed aunt who will be their guardian and protect them. So he asks Hermia to meet him that night at the forest on the outskirts of Athens. Hermia readily agrees to do so:
therefore, hear me, Hermia.
I have a widow aunt, a dowager
Of great revenue, and she hath no child:
From Athens is her house remote seven leagues;
And she respects me as her only son.
There, gentle Hermia, may I marry thee;
And to that place the sharp Athenian law
Cannot pursue us. If thou lovest me then,
Steal forth thy father's house to-morrow night;
And in the wood, a league without the town,
Where I did meet thee once with Helena,
To do observance to a morn of May,
There will I stay for thee. [Act I Sc.1].