Google is making a big change to the way the so-called "Googlebot" crawls the web. The Googlebot is the webcrawler that searches everything on the Internet in order to update Google's enormous database and make sure you get the freshest links possible when you do a search. For the first time ever, the default mobile crawler ID will be Android and not an iPhone.
The Googlebot's smartphone user-agent has been an iPhone, and the most recent version of the user-agent has been pretending to be an iPhone running iOS 8.3. However, on April 18th that is going to change and the Googlebot will begin have a default smartphone user-agent pretending to be a Nexus 5X running Android 6.0.1.
The official explanation for this change is so Google's "renderer can better understand pages that use newer web technologies." Presumably this means web tech like HTML5, which Google has been pushing for a long time. Google also claims that the Googlebot renderer has "evolved" to become more like Chrome than Safari, which does kind of make sense given that the latest stats from NetMarketShare say mobile Chrome makes up almost 43% of the market while mobile Safari is only 33%.
Google claims that 99% of sites should see no effect from the change, and those that do will only see issues if it is looking for a specific Googlebot user-agent.