What is the meaning of Darwin's expression "descent with modification"?

It actually is the phrase with which he referred to evolution, just another name for evolution. Descent with modification means descendants have been modified they have taken certain traits from their ancestors which have been "selected for" or have shown to provide an advantage in procreation. Evolution is the passing of selected traits to offspring, i.e. birds with stronger beaks break more nuts easier, eat more, are stronger, so they procreate more, so more birds in the population will now have stronger beaks, because the strong beak bird had the most offspring. So the new population is now "modified", or to phrase it as "decent with modification", the descendent's have become modified, they have changed to have stronger beaks.

Darwin:

"Darwin's work established evolutionary descent with modification as the dominant scientific explanation of diversification in nature" So Darwin used the above definition (the strong beak bird procreating more) as the main reason for different species in nature (a long beaked bird will get smaller nuts in hard to reach places and also procreate more, so there will now be a diversified ecosystem with long-beaked and hard-beaked birds). The reason there are different species, Darwin proposed, is because specific traits that make animals stronger and better are selected for and these different strings of different selected traits will continue to evolve until they are actually completely different species.

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.

"Descent with modification" means that the species that exist today descended from other, preexisting species, and that species change over time.

Darwin used this phrase to describe evolution. Darwin wasn’t the first to describe the pattern called evolution, but he documented the pattern carefully and related it to the process called natural selection.