Laravel Elixir first came out in October of 2014 and aimed to make automating concatenation, and general file watching easier than setting it up manually. Built on top of Gulp as a wrapper to remove all the difficult parts it has been a popular addition to Laravel.
The next version of Laravel Elixir is changing the underlying system and will be built on Webpack, instead of Gulp. This will replace the plugin ecosystem, and because of such a significant change, it was time to rename the package.
The new name is Laravel Mix, and it should be released along with Laravel 5.4.
If you have been using the default Elixir then switching should be simple and just require updating or moving over your gulp file to the new setup. Of course, complete documentation will be included in the official release.
For more information on Webpack, they have an excellent high level overview of the concepts behind the system.