use of $this->uri->segment(3) in codeigniter pagination

This provides you to retrieve information from your URI strings

$this->uri->segment(n); // n=1 for controller, n=2 for method, etc

consider this example http://example.com/index.php/controller/action/1stsegment/2ndsegment

it will return

$this->uri->segment(1); // controller
$this->uri->segment(2); // action
$this->uri->segment(3); // 1stsegment
$this->uri->segment(4); // 2ndsegment