An Electronic Travel Authorization (eTA) is needed when you travel to Canada by air from a visa-exempt (no visitor visa required) country. IRCC has a simple online checklist you can use determine whether you need a visitor visa or can travel on an eTA. [See IRCC: Find out if you need a visa to travel to Canada]. Note that there are also eTA exemptions. [See IRCC: Find out if you need an eTA]