What is the undertaking as to damages I have to give to get a Mareva injunction in Ontario?

An undertaking as to damages is a formal promise you make to the court, as a condition of getting a Mareva injunction or most other interlocutory injunctions, to compensate the defendant (and sometimes affected third parties, like banks) for any loss caused by the order if it later turns out you weren't entitled to it - for example, if you lose at trial, or the injunction is later found to have been wrongly granted. Courts require this because an asset freeze can cause real, immediate harm to a defendant's business or finances before their side of the story has even been heard, so the plaintiff has to accept the risk of that harm if the order turns out to be unjustified.

Courts will generally want some assurance you can actually make good on this undertaking, not just words on paper - for a significant freeze, that can mean being asked to demonstrate financial means, or in some cases post security, to back up the promise. Because the undertaking is a real, binding commitment that can result in you owing the defendant money later, it's an important factor to weigh seriously before seeking a Mareva injunction, not a routine formality to sign without thought.