Apple now have enabled 2 factor verification for iCloud – 2 factor means you need two things, such as a password and a code sent to your phone, to access your account.
Enabling 2 factor authentication also means that your account cannot be accessed via ironically insecure security questions (i.e., what’s your mother’s maiden name).
Log in and enable it here: https://appleid.apple.com/account/manage/security

Just be aware that you need to keep the Recovery Key in a VERY safe place – if you lose it, and you forget your password, Apple have no way to reset your password. This is good as it means that no-one else can ever reset your password and hack into your account but it also places a burden on you to keep the recovery key (or at least your password) safe as there is no way to reset your iCloud account password without the recovery key.