The column header pretty much says what they are for.
They are based upon
Dublin Core (DC) matadata
Calibre adds a couple for the user
{date} is the date the Record was made
{id} is the serial number Calibre adds to make sure the details match the book saved (If you peeked beneath the skirts
tou will notice they all are
title (##) That ## is the ID
{rating} a place (stars) where users score the book
{tags} Any keyword list you want to assign to help select the book. Common is genre
{last_modified} ANY change made to the metadata used in Calibre