It is the unique identifier for each document in a collection. Compared to UUID, it is smaller (12 vs 16 bytes) but should still be workable because the requirement is just to have uniqueness within the DB cluster.
Uniqueness is guaranteed by including the machine ID, process ID, a timestamp and a counter for entries generated within the same second.
Link: http://www.mongodb.org/display/DOCS/Object+IDs
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