Add tootip to td

Add tootip to td

Hello,

I saw a piece of code for adding tooltip to table cell.
  1. $('#grdList tr td:nth-child(5)').each(function(i) { 

    if (i > 0) { //skip header

    var sContent = $(this).text();
    $
    (this).attr("title", $(this).html());

    if (sContent.length > 20) {
    $
    (this).text(sContent.substring(0,20) + '...');

    }

    }

    });

grdList - table id

td:nth-child(5) - column 5
I don't understand it. Is it an api or plugin?

Thanks for advice.