It's a general emergency alert system, for floods, tornados, major industrial accidents, missing children, etc. It's run by each province, as the matters it covers are ones the provinces have powers over.
Every 4G phone or later has to have software built into it which can receive the alerts. You can't turn it off except by turning the phone itself completely off. It's been very unpopular because you get woken up for alerts for missing children on the other side of the province 1,000 km away when there is no realistic prospect of you being in any position to do anything about it (you're in bed, and you're 1,000 km away). It's like you being in the UK and getting an alert every time something happens in Poland.
A lot of people are unhappy with it and would like to be able to have some control of what alerts they receive and when. However, the groups who concern themselves with whatever it is people are being alerted about see their topic as being something that everyone must know about right now, regardless of how unrealistic it is that it would make any difference.
Meanwhile Ontario has two alert RSS feeds which are supposed to disseminate this information, and the only thing that has been posted to either of them is a single tornado alert last June.