Monzo is a digital only UK bank. It is fully authorised in the UK by the Prudential Regulation Authority (PRA) and regulated by the Financial Conduct Authority and the PRA. Simply put, if Monzo goes belly up, your money is protected. If you are over 18 years old and have a Android or iPhone (as it’s app only), it’s worth signing up, it takes minutes. Especially whilst they are giving away £10 for new customers! Monzo has no fees (other then overdraft charges - which are optional).
You don’t have to use it. You don’t have to transfer your direct debts, salary. You can, but it’s completely up to you. It’s still worth keeping your existing bank account, because it’s digital only, so you can’t pay cold hard cash into their branches, as they have none. It’s also worth having a VISA card, just in case of problems with MasterCard.