What does hook_exit() will do in drupal?

Perform cleanup tasks.

This hook is run at the end of each page request. It is often used for page logging and printing out debugging information.

Only use this hook if your code must run even for cached page views. If you have code which must run once on all non cached pages, use hook_init instead. Thats the usual case. If you implement this hook and see an error like 'Call to undefined function', it is likely that you are depending on the presence of a module which has not been loaded yet. It is not loaded because Drupal is still in bootstrap mode.

Parameters $destination: If this hook is invoked as part of a drupal_goto() call, then this argument will be a fully-qualified URL that is the destination of the redirect. Modules may use this to react appropriately; for example, nothing should be output in this case, because PHP will then throw a "headers cannot be modified" error when attempting the redirection.

Return value None.