Unlocking a mobile phone means you can use a SIM card from any provider in it. For the longest time, most Canadian carriers refused to unlock devices until your full contract term was up and then only for $50. By the time the device was eligible, it was likely obsolete and unlocking no longer makes sense.
Their logic was that by allowing customers to unlock early, they would risk losing money when customers took these devices to other networks and refused to honour their contract. Truth is we have 3 major carriers in Canada (with sub carriers and other owned brands added to the mix), so this isn't a viable strategy.
A new day
As of March 2013, any Rogers customer in good standing with a device that has been activated for at least 90 days will be able to pay the $50 unlocking fee and unlock the device immediately. This is great news and is likely a response to the CRTC investigation into the Canadian mobile phone industry.
Fido (a wholly owned Rogers brand) unlocks at the end of your term for $50 but hopefully that too will change. Telus will unlock devices after the 90 day moratorium for $35. Bell/Virgo will unlock devices for $75. I am not sure of Videotron's policy but have heard they aren't too eager to unlock devices.