Quand ouvre-t-on les cadeaux?

No meaning. It is there for pronunciation purposes only when forming inverted verbs ending in a vowel with il/elle/on.  

https://kwiziq.learnfrenchwithalexa.com/revision/grammar/forming-questions-by-inverting-in-the-present-tense-il-elle-on-forms  

I think That it may have two meanings 
 Quand ouvre-t-on les cadeaux?(when do we open the gifts=because "on" can be  used for "we")
Quand ouvre-t-on les cadeaux?( when  are the gifts opened= because "on" can be used for anonymous subjects in passive sentences

Its just a liaison for pronunciation because there are two vowels together otherwise 

When noun is ending with vowel and subject is starting with vowel we have to add -t- in between them.