It would mean that, in any given industry under federal regulation, union membership would be optional, that you would have non-unionized workers allowed to work beside unionized workers or have no unionized workers in what were previously unionized workplaces.
Right-to-work legislation is the kind of legislation you see in places like Alabama, Mississippi and places where they want to keep unions away, because they don't like the kinds of things that unions advocate for: good paycheques, leave, sick leave and safety and occupational practices that favour workers, protect them and make sure they're well paid.
Conservatives are against that. They're against it in Mississippi, and they're against it in Canada, apparently. It's written right here, black on white.