RH-1 is single family home zoning. Sometimes, you'll see RH-2 advertised because it implies that you could make the building into a 2-unit... emphasis on the word could. I've got a chart on my SF real estate blog, see below.
Also note that there are two RH-1 zoning codes in San Francisco: RH-1 and RH-1(D). The latter is for detached single-family homes only, whereas the former can also have attached homes. Zoning is complex, you may want to take a look at a new site called Zonability, which covers San Francisco (see attached URL).
RH1 is residential single family. I was going to get you a link to a sheet that shows all the code from Sfgov but it isn't there anymore.
There is massive rezonining going on in many parts of the city and that is probably why. If you have any other questions you should take your lot and block number (tax records) down to the building or even better the planning department and find out if your lot is being rezoned.
But RH1 is single family use only RH2 would be 2 residential units.