TGC was developed for the crowd sourcing project of calculating 3D coordinates. Thread: What is the most efficient way to crowdsource the 3D system coordinates. The authors hasn't created its own thread yet, but its discussed and questions can be asked in that thread.
The 3D coordinates of a star sytem X can be calculated by a process called Trilateration. As input, the distances from system X to other (good reference) star systems are used. In an ideal world 4 would suffice but in practise 5 or more are used. After the precision in the Galaxy map was reduced from 3 to 2 decimals (the navigation menu was always 2 decimals) the work got a lot harder - precision is key. Other commanders came up with different 'needle/plane' techniques to overcome this limitation. A whole LOT of effort has gone into those algorithms. Admin note: Not 100% sure which method or a combination of methods TGC is using, but it works.
Commanders can upload distances to TGC using a large variety of tools - see API section at the bottom of the page. When enough (good) distances are present TGC is able to calculate the 3D coordinates for a system. The tool which is created and used by the author is EDSC.
Note: The author already had a domain for EDSC. For various reason he decided to reuse the domain for TGC. Hence confusion. The tool is mostly called TGC in the 3D coordinates thread. Outside of it its often incorrectly referred to as EDSC.
Update: The site and code is not updated since the 16th of December and the author has not posted since then (there). Issues have surfaced and a new tool has emerged, called EDSM. Which is an acronym for Elite Dangerous Star Map. EDSM is on an API level compatible with TGC but offers more funcitonality and API calls. Its also very actively updated. Authors of tools using TGC are very much encouraged to switch to EDSM.