about a piece of code from drupal 3.0.x

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hi all
i don't understand this function .if someone help me explain it ,i
would appreciate
the code:

// invoke hook $hook of module $name with optional arguments:
function module_invoke($name, $hook, $a1 = NULL, $a2 = NULL) {
 $function = $name ."_". $hook;
 if (function_exists($function)) {
   return $function($a1, $a2);

what is it would return  ? why write $function( $a1 , $a2);?

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