What does this message mean?
I got a message saying my Apple ID and FaceTime are being used on another device.
Another device just signed into your account.
If nobody else is authorised to do that:
Make sure you have another method of receiving auth codes like another trusted friend’s mobile
https://support.apple.com/en-us/HT204915
you need to log into icloud.com and make sure the other devices are removed from your account.
https://support.apple.com/en-us/HT204074
More important
change your appleID password.