$_ is a DataRow as you previously highlighted. A DataRow is made up of data items. You can access an individual item using the index operator []. There are two versions of the index operator; one takes an integer and one takes a string. The integer version takes the zero based column index (i.e. 0 is the first column) whereas the string version takes the column name (e.g. "A" if the query was "SELECT A from TableT").

Details of DataRow can be found here.

So what you have got is a collection of DataRows and for each of those rows ($_) you are accessing the value of the first column for that row ($_[0]).

(You're most welcome.)