If you have a cryptocurrency wallet, whether hardware (Trezor, Ledger) or software (Electrum), you may have generated a 12 or 24 word mnemonic phrase. This phrase must be kept secure because it is effectively the key to all of the funds stored in your wallet. Anyone with access to this recovery seed can access and transfer your funds, unless you have a 25th word passphrase, in which case they would require that too.
With this in mind, this is a non-comprehensive list of things not to do with your recovery seed if you want to keep your wallet secure.