What is dbItemID of a tablegrid and in which table we can see its values?

The DbItemID is how MVD associates a specific row in a tablegrid back to the correct row in a corresponding table.
Sometimes you might see 'sqlvalue' used instead of 'dbitemid' - it does the same thing.
Its value is whatever the primary key of the table that you are looking at.  There is no single table that holds all the primary keys of all the other tables - why would you need something like that?
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Absolutely not.  Primary keys and foreign keys are maintained automatically by MVD and any attempt to assign a different value will most likely result in relationships between tables being broken.  Why would you even want to?