If the concentration of molecules is equal on both sides of a membrane, the molecules will move randomly in all directions. This is because there is no concentration gradient to drive movement in any particular direction.

While there will be individual molecules crossing the membrane in both directions, the net movement across the membrane will be zero. This is known as dynamic equilibrium.

Think of it like a crowded room with people moving around. Even though people are moving, there's no overall movement in any specific direction because people are moving randomly.