Here's how it happened:
* Cato, a dangerous tribute from District 2, had Katniss pinned down and was about to kill her.
* Rue, the young girl from District 11 who had formed a bond with Katniss, saw what was happening and shot an arrow at Cato, distracting him.
* This allowed Katniss to get away, and Rue was able to escape as well.
Sadly, Rue was later killed by another tribute, but her sacrifice saved Katniss's life.