What does "studio in-law" mean in the context of a studio apartment?

I’ve been looking at all kinds of apartments on Craigslist and came across that a couple times. From what I can infer it seems to be a tiny, separate living space that’s part of a larger home. Is that so?

What are the advantages/disadvantages or things I should be aware of that I might not think of if I were to consider a studio in-law?