It stands for “unknown,” meaning that someone didn't enter the units (miles or kms) for the odometer reading or was unable to confirm the unit of measure for the currently displayed odometer reading.
This does not mean the true distance traveled is unknown or that the odometer may have been tampered with.
Before you buy a used car, we recommend that you get a CARFAX Canada Vehicle History Report, take a thorough test drive and have the vehicle inspected by a qualified mechanic and/or auto technician to make sure the vehicle is functioning properly.