Why is the kiss considered a monumental moment in The Three Sisters?

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.