The most important thing to remember when dealing with Bitcoin is to keep your private keys safe!
Bitcoins are stored in accounts that require private keys to prove ownership. Typically these keys are stored in your bitcoin wallet by the app. Without the private keys, you can’t prove that you own the bitcoins in those accounts, and won’t be able to spend them. On the other hand, anyone who gets copies of those private keys will also have full control of those accounts, and can spend them even without your approval.
Because of this, it is very important to keep those private keys safe and secure. These keys can come in many forms. They can be a long string of random characters, a QR code, or even a list of 12 or more unique words. Mycelium uses the 12 words to create your keys, and stored bitcoin addresses in what is called an HD Wallet account. It is very important that you keep a copy of those words secure, but also somewhere where you won’t lose them. If you lose your phone, or it gets wiped, and you don’t have a backup, your bitcoins will be lost forever. And if someone else gets access to your private keys they can take your bitcoins. It is also recommended to make more than one backup copy, in case one of the backups gets lost or damaged.