Hotmail security code

I have three Hotmail accounts. I've recently gotten a pop-up when I sign-in asking for or requesting I get a security code. I exited the Hotmail account and tried to sign in again and I got into my Hotmail account. But I believe it asked me originally to supply an email account where a security code could be sent. But I believe they listed the same account. If, in the future, I could not get into the Hotmail account, how could I get the security code they sent to that Hotmail account.
This is very confusing and would appreciate any help in understanding what Hotmail is asking for.
Thank you.