SNN stands for Secondary Name Node. It is also a Master Node. SNN also consist of Fs image as well as Edit logs. In addition to these they also have a checkpoint directory. When the cluster is up and running, upon a set time interval the details from the FS image and the Edit logs of the Name node are copied to the SNN FS image and Edit logs. Here these two combine and the details are put into Checkpoint directory.
The most important use of SNN is as a recovery i.e. while restarting the Name node if there is any dataloss then the details are fetched from SNN. This is a manual process as of now by using get and post methods.