Xapo is a crypto bank in a sense. They are custodians of thousands if not hundred of thousands of private keys which are held by them, and it's their responsibility to keep them safe. Apparently they have an underground cold storage bunker somewhere in switzerland, we have at least picture evidence and im sure there are videos so we can assume that it's all real, im not sure if they've been audited. I would know all of these details if I was choosing them to hold my money obviously, but the idea is that they are indeed a bank. You should also look with massive detail about how much of what you are holding will they cover in case of bankruptcy. Since they are a private company, it could be that you would lose everything in case they file for bankruptcy, so have that in mind.