Majesty is the final tech of the Diplomacy tech tree branch. Its tooltip clains that it allows you to form "Assimilation treaties" with other races, which I assumed means that they unite with you / surrender to you peacefully. However, after researching the tech, I didn't get any new options for treaties, and I couldn't find a new option anywhere else in the diplomacy menu. I have max good relations with multiple AIs (slider all the way to the right), so I guess that isn't the problem.

Then I checked the "Majesty" entry in the tech tree again, and I noticed that it doesn't actually have that small yellow document-icon that you usually see on a tech that unlocks a contract - it just has the green plus-icon for a "diplomacy +1" bonus.

Thus, it looks like the tooltip is outdated, and the tech doesn't actually give you anything apart from that +1 diplomacy bonus - however, in that case, what would be the point of it? Even if +1 is a big deal (the diplomacy stat isn't explained well enough for me to be able to tell), some stat boost seems quite underwhelming for the final diplomacy tech.