The effective nuclear charge is the positive charge experienced by a particular electron from the nucleus that it is surrounding. The distance of the electron from the nucleus coupled with a shielding effect from neighboring electrons causes this value to change from element to element. The general trend is that the effective nuclear charge increases as you move across a period (horizontally left to right) and it decreases as you move down a group (vertically top to bottom). The reason for this is electronic configurations. Moving across a period increases the size of the nucleus but the valence (outermost) shell of electrons remains the same. This means that the electrons are staying the same distance from a nucleus increasing in size, hence increasing the effective nuclear charge. But when you move down a group, even though the nucleus is larger, you are adding an entirely new valence shell of electrons that are further away from the nucleus, thus they have a smaller effective nuclear charge.