Every resident of Canada must file an income tax return in Canada every year and report his/her worldwide income. When you have substantial ties with Canada, you will be filing a tax return in Canada as a resident of Canada. If you are a non-resident of Canada, you may only be required to file an income tax return if you have income from sources in Canada.
A U.S. citizen must file an income tax return in the United States every year, irrespective of their residential status. Otherwise, all residents of the United States must file an income tax return in the United States and report his/her worldwide income. If you are a non-resident of USA, you may only be required to file an income tax return if you have income from sources in the United States.
In the same tax year, suppose you have income from sources in the United States and in Canada. Since you are a Canadian resident and you have to file an income tax return in Canada. And if, based on the Green Card Test or Substantial Presence Test, it is determined that you are also a resident of the United States, then you have to file an income tax return in the United States as a resident of the United States.
In the above situation, you are in Dual residency status for tax purposes. The criteria applied for testing residency status in Canada and in the United States are different. Canada determines your residency status based on the substantial ties with Canada. While US determines your residency status based on either the Green Card Test of number of days present.