A sukkah is a temporary, hut-like dwelling built during the holiday of Sukkot. (In fact, the word sukkot is the plural of sukkah.)
Sukkot, the Jewish festival of booths (a harvest holiday of thanksgiving), begins on the 15th day of the Hebrew month of Tishrei.
It is a mitzvah to build a sukkah and to welcome guests to the sukkah.
The etrog is the citron fruit used as a component in the arba minim, the four species.
It is a mitzvah to build a sukkah and to celebrate in a sukkah.