What agreement do Mrs. Linde and Krogstad come to that makes them both happy? How does their agreement affect the Helmers?

In Henrik Ibsen’s play A Doll’s House, the characters Kristine Linde and Nils Krogstad reunite after many years; they were once in love, but Kristine broke it off and married someone else out of sense of duty to her family. Now they have met again and reconciled and discovered that they are still in love. This makes them both very happy.
Meanwhile, Nils has blackmailed his boss’s wife Nora Helmer out of desperation because he fears losing his job. However, having found happiness with Kristine again, he wants to withdraw the letter he has written to Nora’s husband informing him of Nora’s deceit (she had taken a loan without her husband’s permission, an illegal act at this point in time, but it was for his health). Kristine persuades him to allow things to take their course instead. So Nora’s husband Torvald Helmer finds out about the loan and tells Nora very harshly that their marriage is all but over—until he realizes that the loan does not have to be repaid and relents.
The net result of all this is that Nora leaves Torvald, though, because the nature of their relationship becomes clear to her. She says,

You don’t understand me, and I have never understood you—till tonight.

She realizes that she has only ever been like a doll to him, that he isn’t the person she hoped he was and that she will have to go out into the world and find her own way in life; otherwise, she will not be useful to herself or anyone else. So the renewed relationship between Kristine Linde and Nils Krogstad irrevocably changes the course of the Helmers's marriage and lives.
There are also extensive summaries and analyses of the play on this site, linked below.