Understanding syntax...LASTDATE(table1[table2.dateColumn])

Hi all, newbie learning...
I've been trying to use the reference docs. to fully understand the syntax of the table1[table2.datecolumn], used in a CALCULATE filter, but cannot find any reference to this syntax - it is always 'table1'[columnName] in the ref docs...

I can see what it does but not why it does it which makes me nervous...

Any pointers would be much appreciated.

Thanks

Iain.