Equilibrium of a body is a state in which all the forces acting on the body are balanced (cancelled out), and the net force acting on the body is zero. The state of equilibrium is a very important concept to learn in physics. If the net resultant force acting on a body is zero, it means that the net acceleration of the body is also zero (from second law of motion).
Generally there are two types of equilibrium:
Suppose a block is resting on a floor and two forces of 5N each are acting on it from either side. The forces would cancel each other out and hence the net force on the block would be zero. Since the block is at rest, it will be in static equilibrium.
A block attached to a spring and doing simple harmonic motion (S.H.M.) is a common example of dynamic equilibrium. At the mean position, the net force acting on the block is zero but the velocity of block is maximum, which means that the block is in dynamic equilibrium at that point.