Simple Definition:

Equilibrium is a state of balance between opposing forces or factors. It occurs when the forces acting on an object or system are equal in magnitude and opposite in direction.

For example, a seesaw is in equilibrium when two children of equal weight sit on opposite ends. Another example is a ball staying still, not moving in any direction.

Very Simple Definition:

Equilibrium is when things are balanced and not moving. It happens when everything is equal and opposite.

For example, a teeter-totter is in equilibrium when both sides are the same height with the same amount of weight.