It's just something ArcGIS uses to maintain and track changes to metadata. Nothing you need to set yourself.
According to some old docs  and 
When metadata is created, the following elements are added to the ESRI
section, which is used by ArcCatalog to maintain metadata:
- Esri/MetaID — Metadata ID (ESRI Profile)
- Esri/CreaDate — Creation Date (ESRI Profile)
- Esri/CreaTime — Creation Time (ESRI Profile)
- Esri/SyncOnce — Synchronize Once (ESRI Profile)
The Metadata ID element is intended to provide a unique identifier for
each metadata record. Its value is a globally unique identifier
(GUID) that is generated by the computer.
The Creation Date and Time are the date and time when the metadata
document was created by ArcCatalog or initialized programmatically.
These values will never change. All dates and times that are written to
the metadata by ArcCatalog are recorded in formats that adhere to the
FGDC standard. Dates are written in yyyymmdd format.Times are the
local time and are written in HHMMSSSS format.
The value of the Synchronize Once element will initially be "TRUE".
The value "TRUE" indicates that while metadata has been created,
synchronization hasn't yet taken place.
Upon synchronization, SyncOnce is replaced with SyncDate and SyncTime.
Automatic updates and other editing operations in the User Interface,
or programmatically updates through the
IXmlPropertySetEdit::SetProperty , will add ModDate and ModTime.
Anytime ArcGIS creates metadata, automatic or not, these ESRI
elements will be added.