SOAP (Supplemental Offer and Acceptance Program) is the structured process, run through the NRMP, that lets applicants who did not match, or matched only partly, apply to unfilled positions during Match Week. It replaced the older informal scramble.
SOAP moves fast, over a few days, and requires you to have your documents, letters and program list ready before Match Week begins. Not all applicants are eligible, and visa and certification status affect your options. See the NRMP entry for the wider Match process.