A trusted PC is a computer that you've added to the password reset information for your Microsoft account. This information helps Microsoft verify the identity of the person using the trusted computer.
If you ever forget your password, or if someone gains unauthorized access to your account, it's easier to reset your password from your trusted PC. When you request to have your password reset from a trusted PC, Microsoft trusts that you are who you say you are and doesn't require you to provide any additional information to verify your account.
