Following are the functions of Secondary NameNode:

FsImage which stores a copy of EditLog and FsImage file. NameNode crash: If the NameNode crashes, then Secondary NameNode’s FsImage can be used to recreate the NameNode. Checkpoint: It is used by Secondary NameNode to confirm that data is not corrupted in HDFS. Update: It automatically updates the EditLog and FsImage file. It helps to keep FsImage file on Secondary NameNode updated.

Below is key point for remember :

The Secondary NameNode connects to the primary NameNode and requests a copy of the file system metadata that means namespace image and edit log.
The primary NameNode creates a new checkpoint by saving the current namespace image and the accumulated edit log into a new checkpoint directory.
The primary NameNode sends the newly created checkpoint files to the Secondary NameNode.
The Secondary NameNode downloads the checkpoint files and merges them with the existing namespace image and edit log stored locally.
The Secondary NameNode creates a new, consolidated checkpoint that includes the merged metadata.
The consolidated checkpoint is then sent back to the primary NameNode.
The primary NameNode replaces the old namespace image and edit log with the new checkpoint received from the Secondary NameNode.