You are responding to a thread which is two years old, and a lot has changed in that time. In particular, Alexa does many more things out of the box with smartthings sensors than it used to. So now you may not need to Use AskAlexa depending on exactly what you want to do.
That said, AskAlexa is a community-developed project that uses an Amazon developers account to get you access to many additional features. They have a detailed article in the community – created wiki that explains everything. Very powerful, but set up is also fairly complicated, so you might want to just check and see if what you want to do is already available through the official features.