Backup Node in HDFS is similar to Checkpoint Node. It takes the stream of edits from NameNode. It keeps these edits in memory and also writes these to storage to create a new checkpoint. At any point of time, Backup Node is in sync with the Name Node.
The difference between Checkpoint Node and Backup Node is that Backup Node does not upload any checkpoints to Name Node. Also Backup node takes a stream instead of periodic reading of edits from Name Node.