This phrase indicates a codeshare or partner agreement between two members of the Star Alliance. It means that while you purchased your ticket through Lufthansa (the "marketing carrier"), the physical aircraft, flight crew, and onboard service are actually provided by Air Canada (the "operating carrier"). You will likely see an Air Canada tail fin at the gate, and the flight attendants will be Air Canada employees. It is important to know this for logistics: you must usually check in with the operating carrier (Air Canada) either online or at their airport desk. Furthermore, the baggage policies and onboard amenities (like Wi-Fi or meal types) will follow Air Canada's standards, not Lufthansa's. This system allows airlines to offer more destinations to their customers by "sharing" each other's planes and routes. Always double-check your terminal, as Air Canada and Lufthansa may operate out of different areas in large hubs, even though they are selling the same flight.