We're very sorry for the late response and it's not a stupid question at all!
"SS" stands for "short story". It isn't exclusively otome-ge lingo, but short stories are a common type of store-specific bonus item when purchasing otome games in Japan. These "SS" focus on a scenario often involving the protagonist and one of the love interests.
In Kokorogawari's case, there will be one short story per love interest- so one "set" will have 3 short stories each; 1x Asami x Shinichi, 1x Asami x Yuuya and 1x Asami x Guillam). The pre-order set and the deluxe edition set contain different stories so those who pre-order the deluxe edition set will actually be getting 6 different stories; 2x per couple. =)
Please feel free to ask us any other questions you may have.
Thank you for your interest in Kokorogawari!