When you hold crypto, someone holds the keys that control it. Whether that is you or a platform is the difference between non-custodial and custodial.
Custodial
A platform holds the keys for you. This is convenient and easier to recover if you forget a password, but you are relying on that company to stay secure and solvent.
Non-custodial
You hold the keys yourself, usually via a wallet and a recovery phrase. You have full control, but full responsibility: if you lose the recovery phrase, no one can restore access. Learn more in what is a crypto wallet and public and private keys explained.
Crypto is high risk and largely unregulated in the UK. You could lose all the money you invest, and most crypto is not protected by the FSCS or the Financial Ombudsman Service. This is educational information only, not financial advice. Only invest what you can afford to lose.