If you were a study of Richard Feynman et al, you would understand this concept. What it is saying is that weight and thrust are relatively constant and independant variables acting on the body.
However there is a resultant and balancing vector that is comprised of (classically) Lift and Drag.
However Drag is partially and variably dependant on Lift so it makes sense to resolve these two items as a single vector that is continuously opposing Weight and Thrust (in unaccelerated motion). Simply resolve the vector (at the instant in time that you are interested in) and you have the two (more conventional) items - Lift and Drag.
It is another way of looking at the same thing. But mathematically a single vector of these two co-dependant and variable effects is easier to manage.
Don't sweat this complex math stuff (unless you enjoy the challenge) when there is lots more to understand well before you come to this. From a human / physical perception there is no difference.
MG