What does it mean if my insurer has a duty to defend me in a lawsuit in Ontario?

A duty to defend means your liability insurer has to pay for and manage your legal defence against a lawsuit, usually by appointing a lawyer, once the claim against you could potentially fall within your policy's coverage - even if it ultimately turns out the claim isn't covered, and even if some of the allegations against you clearly aren't. Courts and insurers assess this by looking at the actual wording of the claim against you (the pleadings), asking whether the facts alleged, if proven true, could trigger coverage under your policy - not by pre-judging whether the claim will actually succeed.

This duty is broader than, and separate from, the insurer's duty to indemnify you - actually paying a judgment or settlement if the claim is proven and found to be covered. You can have a defence funded by your insurer for the whole lawsuit while the insurer still disputes, and later denies, that it has to pay anything if you lose. If your insurer denies a defence outright, it's worth having your policy and the claim against you reviewed, since insurers sometimes read pleadings too narrowly when deciding whether a duty to defend has been triggered.