If you know how layer masks work, then you'll understand how clipping masks work as well. Just like a layer mask, a clipping mask allows you to hide portions of a layer. The difference is that a layer mask is a part of the actual layer that you want to mask, while a clipping mask is a separate layer that is used to mask another layer. Another difference is that a layer mask only applies to one layer, while a clipping mask can be used to mask multiple layers.
To learn more about clipping masks, watch this Photoshop clipping mask tutorial: