what does closest function get in jqury code example

Example 1: jquery closest

$( "li.item-a" )
  .closest( "ul" )
  .css( "background-color", "red" );

Example 2: get closest element id jquery

$(".isActive").click(function() {
    var div_id = $(this).closest('tr').find('.display_image').attr(id);
    // ...
});