The evolutionary term descent with modification is most often associated with human evolution, but it is actually a more general term than that would suggest. Simply put, descent with modification means that traits are passed down from generation to generation and sometimes undergo changes or modifications over time. These changes may be caused by Natural Selection or even a mutation of DNA.
The term became popular when Charles Darwin used it in his book The Descent of Man. Since the term was used in conjunction with his idea that humans were related ancestrally to primates, the term descent with modification has become synonymous with the human lineage on the tree of life. However, this is only part of the actual meaning of the term.
On a microevolution level, descent with modification would indicate a change in DNA that leads to a new or different adaptation that could be favorable for a species in that given environment. On a macroevolution level, this term would describe how a species has changed over time and may have even become a new species or gave rise to an additional species through speciation.
Descent with modification can be used to describe changes in a species over generations. For instance, modern day elephants have the traits they have because of descent with modification from their ancestors. Primitive relatives of elephants were as small as a domestic dog at one point. Other points throughout an elephant's evolutionary history had species with long, shaggy hair and extremely long and curly tusks. Through descent with modification, the elephant is now a large animal with smaller tusks and much less hair.