What exactly is pelvic rest and how successful is it for preterm labor?

No intercourse.: It means to avoid anything per vaginam (including sex, tampons, douche, or other devices). It is generally recommended when someone is at risk for bleeding (e.g. From placenta/vasa previa) or preterm labor. It is a common sense approach with not solid evidence of efficacy behind it, however it costs little/nothing and makes sense. Prostaglandins and trauma during sex can trigger labor.