The ability to see a being's energy. To dive into that, it simply means the individual, in this case, Luke, is capable of seeing the flow and movement of energy that exists naturally in living beings.
It's inexplicable, something nonsensical. It was something that none of the Gods or the Primordials were capable of doing. The voice in Luke's head questioned it, which made it clear that he had a skill that wasn't supposed to exist.
Now onto one of the most important questions revolving around it, was Luke the only one with such an ability? Not particularly. He was the only one to have this particular ability, but there existed a certain other individual. Some, just as nonsensical, both skill-wise and mentality-wise.
