This poster depicts a small cat wearing a space helmet. The poster is drawn in a cartoonish style and, as in the name, reads the phrase, "Keep Calm" across the bottom. It is the only poster that is not related to Subnautica or its developer Unknown Worlds Entertainment, in any way.
The poster is likely based on the real-world Keep Calm and Carry On Poster, which was created by the British Government in preparation of World War II to raise morale of the British Civilians.
It can be found in the Canteen, close to the Aurora's Living Quarters.