There are two different kinds of positions to apply for during residency.
Categorical (cat):
These form the bulk of positions throughout programs across the States. Once an individual is accepted for a categorical position, they are destined to stay with a single, specific program for the entire duration of their designated residency. For example; A 5 year residency in the department of general surgery at Massachusetts General Hospital.
Although much harder to attain (especially for International Medical Graduates- IMG's), this is the position that you should strive for. Cats are essentially a guaranteed ticket to a successful completion of residency.
Preliminary (prelim):
Prelims are basically 1 year long paid internships. You are not guaranteed a job after 1 year of work. For example, a 1 year job at Massacheusetts general hospital in general surgery. After that one year, you have to find another place to work. Although there is a chance that the same place that offered you a prelim position will offer you another prelim year, or even a categorical position, its not guaranteed. Most of the time they don't. So then you have to reapply to match somewhere else. If you get offered a position at another place, you dont have to start over again. You simply continue as a PGY2 (second year) at the new institution.
Unfortunately, preliminary positions are commonly offered to IMG's. This doesn't mean always. Some programs can offer you a cat position even if you are an IMG, but this all depends on which institutions you are applying you, and which fields you are applying for.
Finally, there is a transitional year for which one can apply to. However, I do not have sufficient knowledge about these types of positions. I ask that if anyone else does, please post it down below. Thanks.