Labor is a facet, but it’s largely acting on the notion that it’s your class identity and it’s conflict with other classes that fundamentally determines your political interests above things like your religion or gender for instance, although there are forces that act on those things too. So a class interest would simply be a specific political goal which is in the shared best interest of your class. If you work for a living you want better wages, hours, and working conditions, so the more that class organizes around those goals the more power there is to achieve them.