The author talks about what art is, how it evolved, and what impact has reproducibility of art, how it accelerated in the past couple of centuries, and what it means for the actual art. He speaks of art as of having "aura," which would be something like the object being present during the time and baring special significance because of it. In terms of photography, the original has gone through time and has some physical damage because of it etc.
In the age of digital reproduction, all these are lost, and the rate of reproduction is very high, also without any loss of information during the transfer.
As Paul Valery is cited, "'... we shall be supplied with visual or auditory images, which will appear and disappear at a simple movement of the hand, hardly more than a sign,'" this is more than a good forecast and a very knowledgeable assumption. The accessibility of art today is very high and in some way, it is becoming a utility the same way music has (as we can see in subscriptions like Spotify or iTunes Music). The fact that art is easily and effortlessly reproduced has a direct impact on the art itself, and the media through which we consume art also play a big role. Consequently, the definition of art is changing, also the "aura" that Benjamin talks about is different than the one we would see (or try to see) today. It's not that there is no "aura" anymore, but we can no longer see it as a physical artifact on a piece of art.