Translated literally it means secret garden. Which is precisely what it is.
It consists of an ambivalent preference or something that the character wants to keep secret, but at the same time also wants others to know about.
No human being can live completely without making a mistake, or be perfect in the eyes of all. You can’t avoid opening up your heart completely in order to achieve true mutual understanding.
A character’s SG presents their true self clearly. To open up a SG is both an invasion, and an act of love and affection.
Although it was axed due to moral issues, the Sentinel SG’s may have included things like their views on morals, very sweaty situations, and nymphomania.
Also not only the enemy Sentinels, but also allied Servants have SGs.
If you are aiming for a Servant’s ending, you must complete their SG by doing the character-related sub-events.
To digress, the liberties we took with the CGs for Archer’s and Gilgamesh’s SGs had the staff saying, “Just wait a second. We need to cool down and think this over logically. Is this really okay? Shouldn’t we be worried?”
There was a lot of debate about the CGs, but they were kept in since the concept of CCC is about being unable to run from embarrassing things.