That actually is for the holiday of Sukkot, not Rosh Hashana, although the two do have some things to do with each other. Sukkot is the holiday that celebrates the havest.
The Sukkah is a temporary structure in which you are supposed to eat your meals. It must be temporary and I think the roof is also supposed to be open so you can see the stars. The roof also is supposed to be made of organic matter, but the walls can be made of anything.