Effective nuclear charge decreases down a group and increases from left to the right across a period.
Effective nuclear charge decreases down a group because as you go down a group, there are more shells of electrons. This means that the distance between the nucleus and the outermost electron orbital increases; thus the attraction of the valence electrons to the nucleus decreases down a group.
Effective nuclear charge increases from left to right across a period because although electron shielding remains constant, the number of protons in the nucleus remains constant. This means that the nucleus has a greater positive charge, and thus there is a greater attraction between the electrons in the electron cloud and the nucleus (since opposite charges attract).