Page yield is defined as the number of pages you can expect to print with a particular cartridge.  Of course, the amount of pages you can print depends on how much ink you are printing to those pages.  Much like a car MPG (miles per gallon) equation, how many miles you will get per gallon depends on how fast you are driving and other conditions of your driving.  The same applies to page yield for printer cartridges.  The more data/text/images you print to a page, the more ink you use, the lower the amount of those particular pages you will be able to print with a given cartridge.

Therefore, the printer industry uses the same standard for page yield, which is based on a 5% page coverage.  This means that if the page yield expectation for your cartridge is lets say "100 pages", that means you can expect to get around 100 pages, assuming those pages had an average of 5% of the page covered with ink (or toner). 

Attached are 2 pictures showing what 5% page coverage looks like, and a comparison showing what 30% page coverage looks like.  As an example and by comparison, if your printed pages look like the 30% example, that means you can expect to get 1/6th as many pages as the page yield indicates.