It's mostly used by feminists, in protest at becoming 'Mrs' when married. 'Ms' means niether Mrs or Miss, concealing whether the person using the title is married or single. I'm not sure when it was first used, probably the sixties?
It was supposed to put women on an equal footing with men because, like the term Mr, it doesn't allow people to make a judgement based on whether you are married or single
Early 1970s, I think. It's useful when you're talking to or about a woman and you don't know if she's married or not. In the early days people still got very indignant about it but nowadays few absolutely insist on their Mrshood or Misshood. It doesn't 'mean' anything in particular.
One can well see why many women would want to be addressed this way. The trouble is that the word looks and sounds ugly. I have taken, as an experiment, to addressing women whose marital status is unknown to me thus:
Dear Mistress Smith,
So far I have had no comments either way, but it's early days...