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.
It's not a good idea to store more than you want to lose in a "hot wallet" aka a wallet which is currently or has ever been connected to the internet or loaded onto any device that has or may in the future be connected to the internet or any untrusted source!