What does l' refer to

Hi, just wondering what the l' refers to "c'est très souvent que l'on doit changer ou annuler une réservation.

Hi! Don't worry too much ,really... The l' is only a kind of embellishment,like an ornamentation, to get the sentence more musical,more sweet to the ears,as " qu'on " is considered as ponderous,clumsy. Notice that no one would blame you if you omit it ,mostly in a conversation of everyday life. And I swear that even french people can easily forget it. Yours. Julien.

Bonjour, Merci Julien, je ne l'aurais pas pu mieux faire. À bientôt, - Marie-Claire