A cairn is a construction of rocks meant to serve as a memorial or a landmark. The image used for this Acrosstic comes from the SciFi TV series "Lost in Space", which is a Robinson Crusoe variation based on an entire family.
The cairn in this case is special, as it honors both a number of human casualties, and the alien robot that Will Robinson sacrificed in order to keep his family and friends safe. Whether he was right or wrong in that decision is a humongous spoiler, so I won't burden you with it in case you still want to watch the series.