What does the message from the mobile qq security center indicate that the mobile terminal has logged in?

No other meaning, just to remind you that your QQ is also logged in on the mobile phone. This is to tell you that if you didn't do it yourself, it is very likely that your account is logged in from a different place, which means that your QQ has been stolen by others, so you can modify your password. If you log in on your mobile phone, you don't have to worry about this, and you can use QQ to transfer files between your PC and your mobile phone.