Many phones are sold subsidized to a cellphone carrier. To recoup costs, phones are "locked" to only work with that one carrier.
To avoid roaming costs when travelling or take advantage of future competitor plans, many will have their phones "unlocked" so a competitor SIM can be used.
Telus allows phones to be unlocked after 90 days of activation for $35.
If you have an unlocked phone and want to use it on Telus, Telus may offer you a discount.
Just because a phone is unlocked, it does not guarantee it works with all carriers. Biggest example is Australia where their 4G equipment does not work with North America and they sued Apple for misleading advertising.