Clustering in MongoDB - MongoDB High Availability - page 31

Note
What is oplog?
The oplog is a capped collection (a fixed-sized collection) of data that keeps a record of
all data altering operations. The primary node will update the dataset and modify its
oplog , and then secondaries will copy oplog from the primary node and apply the
operations in an asynchronous procedure.
To read more about oplog and its operation, you can visit http://docs.mongodb.org/
manual/core/replica-set-oplog/ .