Prematch offer or what is know as "out of match" position is when a program director offers you to sign for them without the need to go the NRMP match. The ERAS/NRMP system is along story that starts by sending your documents to ECFMG/ERAS and by turn when you apply to programs they will forward your documents electronically to all the programs you applied to. After that some programs will invite you to interview. Classically you have to register with the NRMP and enter there the programs where you did your interviews and on the match date you will know if you were lucky or no. Prematch is the shortcut, you dont need to register to NRMP but directly you get your position early in the season.