Hi Ericj
Basically it means an Agricultural Shelter, depending in what context it's spoken in also a Place of retreat, Shelter and so on, so you would have to suppose at one time it was probably a farmers / farm workers shelter in the event of bad weather coming in, and looking after animals, and had been listed as such in the Escritura.
Maybe over the years it has been used as a permanent house / home with no problems, I have no idea if the Spanish powers to be would re-list it to say a normal dwelling, further in-depth checking by you and a solicitor will be needed on that score just in case you buy it, settle in, only to be told later that it is only to be used as a "Temporary shelter" not permanent living.
Just because someone else might have used it as a house for many years past dont mean you might be able to get away with it, the Spanish law is weird to say the least.