Siyyum means conclusion or finishing, (deriving from the verb som, meaning to place) which comes to mean a marker. The word sprang to prominence last month with the Siyyum Hashas, the culmination of the seven-and a-half-year Daf Yomi cycle, when tens of thousands of Jews round the world celebrated completing the Babylonian Talmud.
Its common to hold a siyyum also to mark more modest achievements in Jewish learning. People will often break open a bottle of schnapps and a box of biscuits after shul on a weekday morning to celebrate completing a tractate of Talmud.
Theres widespread tradition of holding a siyyum after shul on the morning of the Fast of the Firstborn, immediately before Pesach.
The rationale is that firstborns who would otherwise have to fast may eat at the siyyum, in order to participate in the seudat mitzvah, the meal that celebrates a significant religious achievement. Having broken their fast at the siyyum, they are allowed to carry on eating throughout the day.
Ive heard this custom described as a cop-out. Why should you get out of fasting because you hear someone read the last few lines of a talmudic tractate?
I suspect that only someone who hasnt felt the joy of completing a volume of the Talmud could raise such an objection. The investment of intense mental and spiritual effort, which yields a measure of mastery over an area of Torah, and an appreciation of its intricate, interwoven themes, is a big enough cause for celebration, whether the achievement is yours or your friends.