It's kind of an exaggeration: labour that is so hard and heavy you could imagine breaking your back (even if it is not really possible or likely to do this). For "work one's fingers to the bone", imagine that you work so hard that you lose skin, flesh, it's all worn away until there's only the bones of your fingers left. This relates to working "hands-on", meaning you're doing the actual work and not just delegating or organising.

It means doing manual labour for example on the roads, railways, farms. " work one's fingers to the bone" means to work so hard that your fingers become rough and chapped.

This is very difficult, physical labor, such as carrying heavy boxes or doing landscaping outdoors. While it will not actually "break your back," it might leave you feeling sore the next day!