Apple's OS X "Yosemite" brought a lot to the table, and while almost all of it has been good, there has been a stick in the mud. That stick is called "discoveryd", and relates to how the OS handles its networking duties. Since the release of Yosemite, many users have dealt with serious network stability issues, such as disconnections and a doubling of network names, and even full-blown computer crashes.
Apple might have had good intentions with discoveryd, but it's clear that it's causing far more harm than good right now. Fortunately, the company agrees, and so it's decided to pull discoveryd out of the latest OS X beta, 10.10.4, and replace it with mDNSResponder, which had been OS X's earlier networking solution.
It's unclear if Apple has decided to shelve discoveryd for good. It seems likely that the company will continue to develop it and then re-roll out to a future OS X beta for regular users to test out. 9to5Mac notes that so far, none of the discoveryd specific functionality appears to be broken with this revert back to mDNSResponder, which is great news. That could mean that this isn't a mere reversal, but instead an updated mDNSResponder.
Whatever Apple's intentions and implementations, if you've been having networking issues with Yosemite, you may wish to give this latest beta a try.