Brown spots thought to be caused by impurities in paper (e.g.: acid, iron, exposure to humidity, etc.). Foxing is more likely to be encountered in older books. It can be seen anywhere within a book but is commonly heavier on the first or last few pages. The term foxing to prelims is used to describe foxing that is heavier on pages at the front and rear of the book than in the center.
In the first image, the foxing is much heavier at the beginning than in the center. Also note the stain at upper left in addition to the foxing. The second image shows a book where foxing is limited to the exterior of the book -- seen on the dust jacket and also on the outside of the cloth binding.