It's where the Rockbox source code is held;
.. And where all the official builds come from. until now, nobody ever had an official build on his DX50/DX90 I think. But (I hope) soon you will be able to download a very well working build from the rockbox page with all the latest rockbox changes.
To get changes into this repository, every developer needs to upload his changes to "gerrit". In gerrit all other rockbox devs can look at the proposed change and give tips what cold be done better or just deny it. The dev can then correct his proposed change until all senior devs are happy (what sometimes never happens). Then, one of the senior devs merges the change to the repository. Through this procedure we had to go, me first and later ArgelErx. But my change (initial DX50/90 submission which was not very professional) has not been looked at very critical while ArgelErx had to put a lot of effort into splitting his "cleanup" into small changes.
Edited by cholero - 2/9/15 at 9:48pm