FMLA was Bill Clinton's first signature law, in February 1993, after the previous Congress had passed it twice and George H.W. Bush had vetoed it twice. The compromise that finally got it through gave eligible employees up to 12 weeks of unpaid, job-protected leave a year for qualifying reasons — a serious health condition, caring for a spouse, child, or parent with a serious illness, or the birth, adoption, or foster placement of a child.
"Job-protected" is the part that matters. When you come back, your employer has to put you in your old position or an equivalent one with the same pay, benefits, and conditions. They can't fire you, demote you, or quietly cut your hours because you took the leave.
The catch is who actually qualifies. Roughly 44% of U.S. workers don't meet FMLA's eligibility tests — too small an employer, not enough hours, not enough tenure. If that's you, your state may still cover you (CA, NY, NJ, MA, WA, CO and several others have their own broader leave laws), but federal FMLA isn't going to be the answer.