Riess et al's findings showed that galaxies farther away are not just receding faster (as Hubble's Law already described), but that the rate of expansion itself is increasing over time. This discovery suggested the presence of a mysterious force, now called dark energy, driving this acceleration.
This was unexpected because most astronomers at the time thought the expansion should be slowing down due to gravitational attraction between galaxies.
>The farther away that galaxies were, the faster they were receding.
This is already the case in Hubble's law, which says that the velocity is proportional to the distance. Hubble's constant is the proportionality constant and thus the expansion rate.
Riess found that the Hubble constant is not constant. Instead, the expansion rate is increasing over time.
Take for example galaxies running from us at "c". They happen to be exactly at 13.7B light years - coincidence? I don't think it is a coincidence, and some simple logic leads to conclusion that the same galaxies will be running from us at the same "c" say 300M years later - ie. then it will be "c" at 14B light years distance from us - which means that the Hubble constant is actually decreasing.