A brief introduction to Stuck-At Faults was given in the post: Design for Testability: Need for Modern VLSI Design. You might want to go through it first. Anyway, as the name suggests, stuck-at faults manifest themselves if any particular node in the design is "stuck" at either 0 or 1. A plausible explanation for stuck-at-0 (SA0) might be that the particuular node on question has somehow been shorted to Ground (GND at 0V) . Similarly, a node might be Stuck-At-1 (SA1) if let's say it is somehow shorted to the VDD (at logic 1).
In order to detect a stuck-at-0 fault, we would try to excite that node to the opposite value, i.e. 1 and try and see if we are able to achieve that. If we are able to do so, we can safely say that the node in question in NOT stuck-at-0. And vice-versa.
In the below question, we intend to check the node X for a stuck-at-0 fault. Can you tell what input vector (A,B,C) would be need to give to do so?