Of course the extremes like cheating does not count here.
Things you feel you cannot tell your partner because you think it will damage the relationship possibly beyond repair?
Whether it is, that you used to skin animals (e. g. pigeons) for food to survive.
Or that you are into my little pony or transgirl p o r n or you (used to) spend money a few times on only fans.
Or that you have a chronic but hidden on the surface disease or had mental health problems since a long time.
Or that you have some unpaid debts, that you do not want to drag your partner in.
I will not believe you if you say you don't have things to hide from your partner, that you think you can't tell your partner unless there is 100% transparency. That means your partner has fully accepted you with all your quirks, k inks, weirdness, fet ishes and so on.
Also: Not sharing phone, account and computer passwords do not count here. These are cyber security things.