A software solution stack for creating modern apps on web and mobile that embraces cloud and apis. The stack is a collection of widely used technologies where each part comes with a vibrant community. As a whole because of this the stack comes with a large full stack talent pool.
The above summary touches on some of the whys. For a more in depth story read the introducing the SJGAR stack post. "Focus on your customers, on business value and the developer experience. Do more, and keep things simple."
It is fine to substitute one element of the acronym and still call it the SJGAR stack. Likewise adding a new element is also perfectly fine.
Learn: If you are looking to learn more about some of the technologies in the SJGAR stack head over to Full stack open Course which hosts an open and free course for learning React, Redux, Node.js, MongoDB, GraphQL and TypeScript in one go.
Play: If you want to play around with the stack head over to Serverless Stack. Serverless Stack (SST) is, according to their website, an open-source serverless application platform that deploys to your AWS account, helping you go from idea to IPO.