Make the navigation of a single post page to work

In the wordpress codex, the format of previous post link is:
<?php previous_post_link($format, $link, $in_same_cat = false, $excluded_categories = ''); ?>
Respectively for the next post will be:
<?php next_post_link($format, $link, $in_same_cat = false, $excluded_categories = ''); ?>
Example:
<?php previous_post_link('<strong>%link</strong>', '<< Previous', TRUE, '5'); ?>
<?php next_post_link('<strong>%link</strong>', 'Next >>', TRUE, '5'); ?>