Z-buffer is a term used for referring to depth information associated with a surface, whether it is stored in a z-buffer surface or in the pixel format of a RGBZ surface.
The z-value associated with a pixel indicates how deep within a scene that pixel is, the lower the z-value, the closer the pixel is to the viewer. With appropriate testing, only the pixels closest to the viewer need be drawn..
The source and destination z-buffer are simply the z-buffers associated with the source and destination surfaces respectively.