This is a term given to various gentle kinds of energy which can be detected in a laboratory, as well as by "sensitives", and is one which is difficult to explain easily [often terms like "aura" and "chakra" are involved in the description of subtle energy]. Subtle energy, unlike gravity, bright lights and/or sound waves, and hot surfaces, is best described as the difference between still air on a summer day and the very first hint of a stirring of the air, before one could say that there is a breeze or wind. It is like what one feels about three feet above the ground over grass, compared with three feet above pavement. One can learn how to detect and describe the differences, but they are quietly subtle.
Many times, it is like approaching a person in total darkness and detected when that person is only a few feet away by a barely perceptible change in the air, in heat and perhaps in smell which signals that one is close to another. Such energy is definitely not dramatic or forceful in the normal sense of the word. That would be like expecting the feel of a butterfly brushing against one to feel like a slap on the face. If the perception is a hint of a touch or presence, perhaps one so light and indistinct as to make one even question its presence, then such an energy is of the strength described as "subtle". However, with training, it is possible to learn how to perceive this subtle energy in various ways quite dependably. From that point, one can learn how best to use it.