The kiss that I assume you are referring to is between Masha and Vershinin in Act IV. These characters have fallen in love with each other, even though they are both married to other people.
In Act II, Vershinin declares his love for Masha in a brief scene in which the two are alone onstage. The play is actually crowded with characters, and a two-person scene is not a very common occurrence in it. A few scenes later, in Act III, Masha declares to her sisters that she and Vershinin are in love with each other, much to her sisters' horror.
So, the kiss comes after all of this, not during. Vershinin is in the army and leaving in Act IV. He is saying goodbye to Masha and Olga and, in my stage directions, there is "a prolonged kiss" between Masha and Vershinin.
It is significant in these ways:
- It comes not in a moment when they are alone onstage, but when Masha's sister Olga is also there.
- It does not signal the beginning of a love affair, but rather the ending of it. Vershinin will leave, and by his words to Masha, it seems clear that he does not intend to see her again.
- It is the only kiss that they share in the play.
Up until Vershinin's departure, it appears that Masha's affair with him has been purely intellectual and emotional. It is not until the affair is over that they share a kiss. This irony is at the heart of this action's importance.