This is a concept all users should understand. Since Bitcoin has no central authority like a bank everything depends on the security and accessibility of the “Private Key.” Bitcoin is basically a database that is shared via peer-to-peer connections. All the bitcoins are in there but to access them you need the “private key.” The private key must stay private, anyone who has that private key can access and spend the bitcoins ether now or sometimes in the future. Also, if the key is lost or not accessible then the bitcoins are not accessible. If there is a disaster, fire, flood, etc. and the backups are lost then the bitcoins are gone forever. If something happens to you and your family does not have the private keys then the bitcoins are gone forever. When hackers try to steal bitcoins they try to steal the private key. (It is not covered here but receive-only wallets can be used that do not contain the private keys so the private keys can be kept offline, away from hackers).