Easter Eggs are hidden references to other games or pop culture.
Natural Selection 2
The Natural Selection 2 Poster can be found inside the Wreck in the Grassy Plateaus and in the Aurora, and can be placed inside any Seabase. There are many other objects from Natural Selection 2 in Subnautica as well.
The line occasionally said by the PDA after activating either Data Terminal in the Quarantine Enforcement Platform, "Resistance Is Fertile," is a spoof on the Borg's famous line from Star Trek, "Resistance is futile." Shortly after the PDA says this, jazz music will start to play. This Easter Egg was added by Simon Chylinski after Scott Thunelius jokingly suggested the interaction.
A video of this can be found here: https://youtu.be/ac4wlJoTowE?t=44s
When the Player fabricates the Survival Knife for the first time, the Dialogue says: "Weapons were removed from standard survival blueprints following the massacre on Obraxis Prime. The knife remains the only exception." Obraxis is an animator who worked on Subnautica.
There is a Data Download mentioning the hive mind of Strader VI. Cory Strader is another Subnautica developer.
In the PDA dialogue for the Stasis Rifle as well as in one of Paul Torgal's journal entries, a creature called a "Starwhal" is briefly mentioned. "Farming Starwhal eggs" is also mentioned in the Captain's Log. This could be a possible reference to the game Starwhal where players must control flying narwhals with the goal to pierce each other's hearts with their horns.
There is a chance for the Cyclops AI to say: "You are the best captain on the planet, I'm not even squidding." when the Player has entered it, rather than its normal line. This refers to the Player being the only captain on the planet, and a pun on the word "kidding".
Two Alien Relics, the Alien Rifle and Doomsday Device, are reused assets from the game Future Perfect, another game made by Unknown Worlds Entertainment. The Alterra Corporation is in this game and several others as well. It is not known if this is simply an Easter egg or an attempt to connect two games.
This easter egg is found in Alien Relic 7. CHBCLive was a forum user who made Chinese translations since the farming update. He died battling cancer before the game was completed, and this easter egg was added in his honor.