What is a one-time password? Can't access my Square Enix account

I signed up to a SE account some time ago, it was the only time I logged in their account system. When I tried accessing it again yesterday to pre-order a game from their store it asked me to login with my SE account, which requires 2 passwords? I'm sure I signed up with 1 password, I even

Can't access my account? : r/ffxiv

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