?

Well I have PSP 8 but, the definition of mask is pretty much the same

Mask- A mask layer covers parts of another layer completely or with varying levels of opacity.

That means like one main pic than other pics joined as a background or another part of the picture using different opacity levels between them.

If my explanation doesn't make sense I suggest you go to the PS site they might have some help tips there. Or search on www.google.com