Concurrent forces are three or more forces acting on a body at the same point of application.
If 3 concurrent forces are in one plane and are keeping the body in equilibrium, then the forces can be represented by sides of a triangle.
Then Force1 / Sin A = Force2 / Sin B = Force / Sin C
A, B, C are the angles between forces. A between Force2 and force3. B between force 1 and force 3.
When a load is suspended at P, the middle of a string which is tied on both ends to nails fixed in wall at the same height, the forces of tension and weight are concurrent forces acting at P. Problem is solved using the above equalities.
If the body is not in equilibrium and is moving with an acceleration, then the net force of the concurrent forces causes the acceleration.