These are bonus posts from the story (The Experiment). They generally provide background information relating to the story and the universe in which it takes place.
The term “omake” is borrowed from animé, and basically means “something extra”. Unlike story posts, omake posts do not exist within the fictional universe of the experiment. Some omake may be subject to future updates.
Omake rants are rant pages that accompany omake. Because the omake themselves are out of universe, the omake rants are essentially just regular blog posts, but they include information relating to the omake.