The project in question is Project Maven, a research to develop computer vision algorithms that can analyze video recordings from drones. With this technology, analysis of video results from drones can be accelerated by way of classifying objects and people.
The resignation is a step they took after the protests of thousands of Google employees three months ago did not seem to get a response from Google. They fear that this technology will be misused, and used to do offensive things.
Namely used to determine the target of air strikes by drones. According to these protesting employees, it should be human beings who are decisive for such deadly attacks, not artificial intelligence.
Google has participated in Project Maven since April 2017, but unfortunately it is not revealed what Google is doing in the project. But in its official statement Google called them giving access to software TensorFlow to the US Department of Defense.
A number of employees who resigned mentioned that his boss is no longer transparent related to the work on the project, nor heard any more suggestions and protests made by the employees.