Here's why:
* Dynamic: The movement of molecules across the membrane doesn't stop. There's still constant movement, just an equal amount going in both directions.
* Equilibrium: The concentration of the molecules on either side of the membrane remains stable because the rate of movement in and out is equal.
Example: Imagine a beaker divided by a semi-permeable membrane. One side has a high concentration of sugar, and the other side has a low concentration. Initially, sugar molecules will move from the high concentration side to the low concentration side, driven by the concentration gradient. As more sugar moves to the low concentration side, the gradient decreases. Eventually, the rate of sugar moving from high to low becomes equal to the rate moving from low to high. This is dynamic equilibrium - the concentrations don't change, but the movement continues.
