It could be any combination of numbers, capital letters, small letters and signs within 6-20 characters.

To be safer, you should not use regular password such as 123456, 654321, 666666, 888888, abc123, qwerty, your phone number, your birthday. You should use such as Gklfd_157, aM%89712& etc. And you should change your password regularly and make a copy of it in case of forgotten.