Well.. to understand the value you need to have a bit of knowledge on how microservice architectures work and why an API Gateway (like Kong, which is the most popular OSS solution for that) is a fundamental part of it. ;)

I suggest to start from here to get up to speed: http://microservices.io/patterns/apigateway.html

From a cursory look, it's a set of Lua scripts that augment Nginx with features like authentication and such, so that you can avoid implementing them in the actual services.