By storing the 25 word mnemonic key in a secure location, you have a backup of your private keys and hence all of your Moneroj. Sharing this 25 word key is the equivalent of allowing another person complete access to your funds.
A 13 or 25 word phrase used to backup a Monero account, available in a number of languages. This 25-word phrase (13 words in the case of MyMonero) has all the information needed to view and spend funds from a Monero @account.