Abbreviation of the Traffic Signs Regulations and General Directions. 

The TSRGD sets out the design of official traffic signs that can be placed on or near roads in England, Scotland and Wales, and also sets out the conditions for their use. Essentially, it specificies what signs and markings should look like, and how they should be used.

Now updated in 2016 - Guidance note here. Sustrans have a brief technical summary of the 2016 changes.