Imagine a seesaw, you apply 100 N upwards on the left side of it, and on the right side, you apply 100 N down.  Thus, F_y = 0, and since there's no horizontal force acting upon it, F_x = 0.  But it still moves:

This is because the forces are not acting on the same line, not collinear. 

But if 100 N was acting on both sides of the seesaw and assuming the seesaw is supporting such force, it would an equilibrium system.   This is where we will take a detail look into rotational equilibrium and torque ().  

Have you ever try opening the door on the hinge side?  You need to push harder than if you push at the knob.  This is because the distance from your hand to the hinge (pivot point) is relatively small compare to when you push from the knob.  Product of the perpendicular distance drawn from the pivot point to the point of action where force is applied and the force applied is called torque ().

When writing torque equations, you must choose the center of rotation first.  It can be chosen anywhere you want; however, you will find that in solving equilibrium problems, wisely chosen centers of rotation can make getting the answer more simple.