C++ function slash operator lambda expression

in fact I don't know how to be very precise.

Today, I browsed the following page:

http://ift.tt/1s95s66

I found the following syntax:

GET / _hello = [] () { return D(_message = "Hello world."); }

I found "GET" can be a function by lambda expression, but I cannot figure out what does "/" and "_hello" mean here, and how they connect to something meaningful.

Also, what is that "_message = "?

BTW, my primary C++ knowledge is before C++11.

I googled quite a bit.

Could any one kindly give an explanation?