It's a standardized convertible note instrument, designed to replace convertible debt. Convertible notes are what you raise in a widely syndicated round before you have a firm valuation (ie, in your seed round). It's very easy to "sell" someone a convertible note, whereas an institutional priced round involves huge amounts of due diligence and legal fees. Convertible debt became the sort of default way seed rounds worked, but because they're technically debt they have maturity dates, which are logistical problems for founders; SAFE's just convert into equity directly.
Broadly syndicated convertible note seed rounds are a relatively new innovation; what preceded them was "friends and family" rounds of $50-100k followed immediately by an A round, which was a galactic pain in the ass for founders trying to get their companies up on their legs. A seed lets you confirm your hypothesis about PMF without doing board meetings.
The C.W. is that seed rounds today, no matter who's doing them, are done with SAFEs.
