TL;DR - This is the [in]famous "dancing baby" case in which Universal Music issued a takedown request for a home video of a baby dancing to Prince's Let's Go Crazy. The notice resulted in the video being taken down and then, eventually, put back up after a flurry of DMCA counter-notices. The creator of the video, Lenz, then sued Universal Music for making a bad faith DMCA takedown request. (DMCA allows suits for "any damages" arising from a material misrepresentation in a DMCA takedown notice.)
The DMCA requires takedown notices to include a "good faith belief" statement that that the targeted work "is not authorized by the copyright owner, its agent, or the law." The opinion essentially holds that if you don't think about fair use, or if you know about a viable fair use defense, then this "good faith belief" statement is actionably false.
Note that this standard is quite high. If the copyright holder can show that it formed a subjective belief that the targeted work is not protected by fair use (even if that belief is pretty crazy), then it's not liable.
So, an interesting opinion, but it's pretty hard to see how a properly-represented defendant could lose under this legal standard. It may, however, limit the use of certain automated approaches to finding "infringing" content. While the opinion points out automated ways that a copyright holder could reasonably identify works that are not likely to be protected by fair use (such as where all of the audio and video perfectly matches the copyrighted work), human review will likely be necessary in many other situations, and these humans may be far more cautious than they used to be for fear of an eventual class-action lawsuit.
Opinion - http://cdn.ca9.uscourts.gov/datastore/opinions/2015/09/14/13...
Ars Technica Coverage - http://arstechnica.com/tech-policy/2015/09/eff-scores-a-win-...
The video - https://www.youtube.com/watch?v=N1KfJHFWlhQ