The Original Source fields will give you information about how the lead went from Anonymous to Known in your system. Possible values would be things like the name of the form they filled out or the page where that form was.
You can see other possible values by creating a smart list in your instance with filters like Original Source Info > is not empty.
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Here is a link to documentation article that describes the system-managed fields:
It also lists possible values for Original Source Type and Registration Source Type. The fields Original Source Info and Registration Source Info will contain additional detail for each type.