Meaning of code

Could anyone please tell me the meaning or implication of each line of code? Thanks
$excerpt = preg_replace(” (\[.*?\])”,”,$excerpt);

$excerpt = strip_shortcodes($excerpt);

$excerpt = strip_tags($excerpt);

$excerpt = substr($excerpt, 0, 250);

$excerpt = substr($excerpt, 0, strripos($excerpt, ” “));

$excerpt = trim(preg_replace( ‘/\s+/’, ‘ ‘, $excerpt));