What you are looking at is a process that uses biochemical indicators to stain or indicate certain parts of the specimen.
In a single stained specimen, like this one, you are looking at the calcium in the bones that has been indicated. You are actually viewing the entire skeleton, held together by the muscles that have been treated with a proteolytic enzyme, rendering them transparent. Basically, it's a lot of science and chemistry. Once the specimen is "stained" and "cleared" it is stored in glycerin. The reason for this is more science. If The speed of light travels through an object at the same speed that it travels through what the object is stored in the object is rendered invisible.