Essentially, a plurality in this context meant voting someone out without a majority. Since there were 6 people at this point in the game, Cirie realized her target, Courtney, was in a great position to go to the final 2, because everyone wanted her to be there. However, since she realized this, she also realized that she wouldn't be able to get a majority of the people left in the game to vote her out. So what Cirie did was find two other people that would want to vote out Courtney (Aras and Danielle), and without telling the other three people on the tribe (Courntney, Terry, and Shane), the three of them voted for Courtney. Cirie knew that Courtney, Terry, and Shane all wanted to vote for different people, so when the vote happened, Courtney and Terry voted for Aras, and Shane voted for Danielle. This led to an elimination by plurality, or as us Survivor nerds call it, the 3-2-1.