An electronic Travel Authorization or an e-TA, for Canada is an electronic travel document issued for the nationals of visa-exempt countries. The travelers who are visa-exempt (except for the US) must apply for an electronic travel visa in order to enter Canada if they are traveling by air. It is an electronic document that permits you the cross the Canadian borders. The document acts as a primary source of information to the government to know who is traveling to Canada and from where. It is not available in any physical form, rather it is attached with to your passport.
If you get a new passport, you need to get a new eTA. Having a valid Eta permits you to travel to
Canada as often as you want, but only for a short period of time (for up to six months only). You need not to apply for an eTA for travel within Canada.