You are familiar with passwords, your secret login credentials. Account Keys are longer and randomly generated which adds security.
Because an Account Key is longer, you don't need to worry about adding numbers, symbols, and mixed-case letters to make it strong. The randomness substantially increases your account's security.
When you sign up, Peerio will automatically generate an Account Key for you.
Peerio uses 32 character Account Keys to achieve an estimated 120-bits of entropy, which is estimated to cost over $1B (US) to crack in a year. Peerio also uses scrypt to make cracking even more costly in both time and money.
It is important to write it down on a sheet of paper or print it out from Peerio on your computer and store it securely. If you lose your Account Key, you lose your account!