@Roberta,
It's the only other tie-breaker besides the fire-starting one. If there's a tie vote, a second vote is taken. If there's still a tie, rocks are placed into a bag (one for each player voting, minus the holder of the immunity necklace, who remains safe even though he/she was allowed to vote initially). All of the rocks are yellow with the exception of one, which is purple and the unlucky person who chooses it goes home immediately. The yellow rock holders are safe.
I don't remember it happening very often. In Marquesas, they didn't take a second vote. Jeff told them to huddle and choose among themselves who was to go home or face the rock pull. They couldn't decide (it was still split) so they did the rock pull and a player whose name wasn't even written down in the initial vote pulled the purple rock and was sent home, while the two who were targeted were safe.
I liked the first few seasons when the tie-breaker was much simpler. They just used the number of previous votes against the tied players (from past tribals) and the one with the most went home. They may have developed the rock pull to discourage the contestants from engineering ties. Seems vaguely unfair to me.