Consider two identical charges placed distance 2d apart, along the x-axis (figure). The equilibrium of a positive test charge placed at point O midway between them is

 

If positive test charge is displaced along x-axis, then net force will always act in a direction opposite to that of displacement and the test charge will always come back to its original position. But if test charge is displaced along y-axis, it will never come back to its original position and will fly away along y-axis.