If you are a U.S. citizen, and you have any foreign financial accounts you may have to file an FBAR (Foreign Bank Account Reporting) tax form.
The form is FinCen 114. Foreign means foreign from the U.S. perspective, so a bank account at an institution here in Canada would be a good example.
The rules surrounding this are very complex. Even having signing authority on someone else’s account, is enough to require that you report that account to the U.S. Things you may not think of life a life insurance policy, or a private pension fund through your employer, may also need to be reported.