to use the analogy we used in class (it's a quite rigorous honors chem class, but we have a lot of fun...after the teacher explained shielding, one of our students explained it like this
You have a piece of cake. Mmm. cake. The cake is the nucleus. You are an electron. you want to GET TO THAT CAKE and eat it. But wait! You see some dirt on the cake. You still want to eat the cake, it still looks tasty...but not so much as before.
Basically: The nucleus's + charge attracts electrons with - charges. The charge "felt" by valence electrons is called Effective Nuclear Charge, because it is dampened by shielding. Shielding occurs when there are one or more electron shells (the dirt) between the nucleus and the valence electrons. The more levels in between the nucleus and the valence electrons, the higher the shielding. Lots of shielding leads to a bigger atom because the valence electrons are not attracted as strongly (Effective Nuclear Charge) to the nucleus as the core electrons are, and therefore tend to spread out further from the nucleus.