terminology - What does it mean to shift equilibrium?

What does it mean to shift a chemical equilibrium? For example,

the equilibrium shifts to the left …

I don't understand that.

Answer

Probably you are having problems with Le Chatelier's Principle.

Suppose you have an equilibrium established between four substances $\ce{A}$, $\ce{B}$, $\ce{C}$ and $\ce{D}$.

What would happen if you changed the conditions by increasing the concentration of $\ce{A}$?

According to Le Chatelier, the position of equilibrium will move in such a way as to counteract the change. That means that the position of equilibrium will move so that the concentration of $\ce{A}$ decreases again - by reacting it with $\ce{B}$ and turning it into $\ce{C + D}$. The position of equilibrium moves to the right. Of course, this assumes that there is still some amount of $\ce{B}$ left in the reaction vessel.

For better understanding, refer this.