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.