What did katniss witness in the square does do to stop it?

You're likely thinking of the scene from The Hunger Games by Suzanne Collins where Katniss witnesses the Reaping in the town square.

What Katniss witnesses:

* The drawing of tributes: The names of children aged 12-18 are drawn from a bowl, selecting two tributes (one boy and one girl) from each district to participate in the Hunger Games.

* Primrose Everdeen's name is drawn: Katniss's younger sister is chosen to participate in the Games.

What Katniss does to stop it:

* Volunteering as tribute: Knowing the danger her sister would face, Katniss immediately steps forward and volunteers to take Prim's place as tribute.

This act of self-sacrifice is a powerful moment in the book and sets the stage for Katniss's journey through the Hunger Games.