What is syncing

I use MyCloudEx2 for all my files on a network.
I have mapped logical drives to my pc’s.

So what is syncing?

I have not found an explanation on the KB?

I use Good Sync, it’s one of the Apps in the WD EX2 Ultra local interface. It’s a paid subscription of about $30/yr.
I synced my Desktop HD to the EX2 Share File, then I synced my Notebook to the same Share File.
Now every file & folder in Documents is on both computers hard drive.
I also have access to the files thread My Cloud.