Hash value is a digital signature for each file. By comparing this signatures Air Explorer can know if the files are the same so Air Explorer in that case does not updated this file while synchronizes.

There are different hash functions and each cloud uses one of them. The hash comparison only can do with clouds that use the same hash.

Hash comparison:
-md5: local, 4shared, Google Drive, Amazon, Baidu, pCloud, Opendrive.
-sha1: local, Box.com, pCloud.
-sha256: local, Mediafire.
-No hash: hidrive, Dropbox, OneDrive, FTP, WebDAV.

You can check more information here:
-Synchronization feature in Air Explorer
-Sharepoint file manager and synchronization tool
-Synchronize between folders in your own PC with Air Explorer
-“Save and load synchronizations” in Air Explorer