The Greenhouses are where the Herbology classes are held. There are at least three and they are located on the grounds of Hogwarts. They are numbered based on the plants’ level of danger that are grown within them (i.e. Greenhouse One has the least dangerous plants and Greenhouse Three has the most dangerous).
"Three times a week they went out to the greenhouses behind the castle to study Herbology, with a dumpy little witch called Professor Sprout, where they learnt how to take care of all the strange plants and fungi and found out what they were used for" (PS8).