Salesforce have been dangling DX in front of source-control-hungry developers for some time now. And now it's finally in Open Beta!

This mean that the source of truth for development projects can be Git rather than a SF org. Which means we can finally do branches and releases properly.

In the past all your code had to live primarily in a SF org, and then be copied to Git if you wanted to. The only way to "build" that code and run unit tests was to have an SF org with the right metadata set up. Now, we can store the org configuration in a file which we also keep in Git. Then, to run our build, we can spin up a "scratch org" from the config file, run the tests, and then allow the scratch org to evaporate. 

So, the code can really have an independent life in source control and be built/tested direct from there.