It's a tourist attraction.
Basically a very big greenhouse full of plants and butterflies. The butterflies can't get out. It's a building rather than a 'farm', although they are called butterfly farms. It's unusual to have just the butterfly farm and nothing else; they usually form one attraction at a place with other things going on.
A butterfly farm (think building) is different to a farm that has butterflies (wild ones) on it. You need to specifically state 'butterfly farm' if appropriate as otherwise people will think of the second situation.