I received an email from Microsoft that I don’t understand the contents.

Hello,

It means that someone tried to access your account, and the Security information was replaced. This is the recovery Phone number or email address you use to verify your identity.

So, sign in with your Microsoft account on the security settings page here-

Use a private browsing tab on any web browser and go to this link -

https://account.live.com/proofs/Manage

• Sign in with your Microsoft account and enter your password.
• If you are asked to Verify Your identity, use another option to verify your account and then sign in.
• If you see an unknown email address (@gmail.com), remove it from your account.

Then, enable two-factor authentication for added security and Setup Microsoft Authenticator as the verification method.

To do that,

• Select 'Add another way to sign in to your account'.
• Select Use an app, then complete the authenticator verification.

If you are having trouble signing in to your account on the security settings page, let me know.