“Bona-fide” sub-contractors are generally deemed to be contractors who work without your direction. They hold their own insurances and will usually provide their own materials and tools.
An example might be a specialist Demolition Contractor brought in to demolish a building as part of a larger contract. Although Bona Fide sub-contractors might carry their own insurances, their activities could lead to a claim being lodged against you.
For example, they could do something outside the scope of their insurance cover that leads to a claim or they may not have paid their premium.
Additionally, it could just be that a third party decides to make a claim against you for the negligent actions of one of your bona fide sub-contractors (rather than against them directly), which you will need to defend.