What does the term equilibrium refer to?

It depends on what subject but usually it means the balance. In physics, it would be when (for example) the forces acting on an object are equal and the object is therefore in equilibrium. In Biology it would be the balance of the conditions of the body, water levels for example.

Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere. the winds blow from west to east in jet streams but the flow often shifts to the north and south. jet streams follow the boundaries between hot and cold air. the earth's rotation is responsible for the jet stream as well.