“Hidden faults” means faults or defects of which the purchaser was unaware – and could not reasonably have been expected to discover – when making the purchase. For the vendor to be held responsible for a hidden fault, the fault in question must result in the property deviating from what the buyer could reasonably expect.
For example, as a buyer you cannot reasonably expect a 20-year-old pool pump to be in perfect working order; you should realise that you will probably have to replace the pump in the immediate future.
In practice, the types of hidden fault for which the vendor is likely to be held responsible relate to defects in the design or construction of the building. However, as properties are bought and sold "as viewed" (cuerpo cierto), the chance of successfully filing a claim for compensation is very limited.