The backup is a copy of your private key, that uniquely links a user with a wallet and it is necessary to restore your wallet and access your funds in case of hardware failures. It is represented by a mnemonic phrase of (usually) 24 words. Setting a backup is fundamental for the security of your wallet, as it is the only way to recover your funds in case your wallet will get stolen or lost. The backup can be noted both in physical and digital form. Make sure no one is looking at it while you write it down. In the unfortunate event of someone becoming aware of your backup, he could then access your wallet and empty your funds.