"ubd" is the Ubiquity Server Process. It is primarily used for "Mobile Documents" which is an iCloud feature for synching files and documents from your iOS devices to the cloud. There are no configuration options to ubd, and users should not run ubd manually.
A log file can be found in ~/Library/Logs/Ubiquity/[username]/ubiquity.log
"ubd" is known to sometimes cause high CPU usage (you can often tell because the fan is making a loud noise even if you're not using your Mac heavily). You can check how much CPU "ubd" is using from the Activity Monitor.
If this is happening on your Mac you can try to kill the process from the terminal although sometimes it 'comes back'.
Alternatively, try deleting or removing your iCloud account from your Mac, restart your computer and re-add it again.
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