Replication factor is same as the number of copies of a file on HDFS. If we set the replication factor of 1, it means there is only 1 copy of the file.
In case, DataNode that has this copy of file crashes, the data is lost. There is no way to recover it. It is essential to keep replication factor of more than 1 for any business critical data.Read the full book at www.amazon.com