When you apply to residency, if you are applying to a program with advanced (PGY-2) positions, you ALSO apply for prelim positions at the same time.
On the Monday before match, you could then get several different types of emails:
1 - Congratulations, you have completely matched! This means you matched into an intern/prelim/PGY-1 spot AND an advanced position.
2 - Congratulations, you have matched into an advanced position. This means that next year, you have a guaranteed spot in an advanced program but you didn’t match into an intern/prelim/PGY 1 spot. You would then need to scramble into one of these.
3 - Cogratulations, you have matched into a first year position. (I believe this is what your question is really about).
This means you did NOT match into an advanced program. You have several options including applying again the next year and having a year gap between your PGY1 year and your advanced position, getting an advanced position that starts “on time” for you because someone dropped out, continuing in the specialty your prelim year is in (for example, the internal Med program says they would keep you for a full internal Med residency), quitting medicine, etc. All of these options have different rules and different timelines depending on the situation and institution.
4 - You have not matched. Self explanatory.