It's working, but I don't know how its doing it

I've managed to copy some JS over into my doc, and I've got it working. But I dont entirely understand how its doing it.

It's a search function to match with data in a table and hide any rows that don't match.

But I don't understand the active line of code that actually searches and matches. please can someone explain it?

$('#searchBar').keyup(function() {
  searchFunction($(this).val());
});

function searchFunction(value) {
  $('#results tr').each(function() {
    var found = 'false';

    $(this).each(function() {
      if ($(this).text().toLowerCase().indexOf(value.toLowerCase()) >= 0) {
        found = 'true';
      }
    });

    if (found === 'true') {
      $(this).show();
    } else {
      $(this).hide();
    }

  })
};

It's this line I can't get my head around:

if ($(this).text().toLowerCase().indexOf(value.toLowerCase()) >= 0) {
  found = 'true';
}

I understand how it changes the variable to true, but I don't understand how it matches the data in the Table row with the Value fed in.