What exactly is a trust root?

A trust root is the foundation of most network encryption and authentication, among other uses. Certificates legitimately issued to and associated with websites, network services, individual people, and other such services can be cryptographically traced back to root certificates present in the trust store. If the certificate is not traceable back to the local trust store, or if there is no certificate presented, the network connection is not trusted.

Certificates are unrelated to your particular quest.

If you suspect your Apple ID is compromised, change your password, enable two-factor authentication, and related steps:

… If you think your Apple ID has been compromised - Apple Support

More info on securing your Apple ID and your devices:

… Personal Safety User Guide - Apple Support