The idea that as each organism or group of organisms feeds on a body, it changes the body. This change in turn makes the body attractive to another group of organisms, which changes the body for the next group, and so on until the body has been reduced to a skeleton. This is a predictable process, with different groups of organisms occupying the decomposing body at different times. This technique allows you to tell, by the age and specific species living on a corpse, how old the corpse is.
Succession and forensic entomology also show if the body has been moved.