Hi! Non-concurrent forces are two or more forces whose magnitudes are equal but act in opposite directions with a common line of action. This basically means that When the forces acting on a FBD do not intersect at a common point, the system of forces is said to be non- concurrent. On the other hand, in a concurrent force system, all forces pass through a common point. If by non plane you mean non co-planar, this force system refers to the number of forces which do not remain in same plane. Hope this helps!