It can be used. in a metaphorical sense, to mean a thing that is a possible solution. Say, for example, you go to the doctor with a fever and horrible cough, and there is blood in what you cough, someone could say that pneumonia or lung cancer are possible candidates for what is making you sick. The meaning is that there are multiple possibilities that we need to test to be sure about which of the candidates is what actually is the problem, but we think it is probably one of these candidates.
The idea of a thing being a candidate is that it is a possible solution to the problem, something that is worth looking at in more detail to see if it is the solution, or not.