A generic term used to describe an utterly bewildering array of dress codes in the British Army. Barrack Dress is the required turn out between Working and Undress, i.e. Number 2 Dress.
This is rather simplistic and fails to take in to account the myriad vagaries of regimental requirements: regimental pattern shirts, pullovers, shoes, brogues, trews et al. These requirements are invariably WAGIs at the behest of the CO and enforced with much enthusiasm by the RSM.
Quite who dreams up these dress codes has never been properly established, but Barrack Dress makes total mockery of the word 'uniform', as Barrack Dress is usually anything but.
For a general idea of dress requirements, see here. And this is just ONE regiment!