This description is provided by Olive Thexton of Fylde Branch

BT - Bishops' Transcripts - are copies of the annual entries into the parish registers sent to the local Bishop from about 1598. 

For some parishes, the BTs have survived where the original parish registers have been lost. 

Be aware that these are copies of the originals and not the originals themselves. 

In parishes where both the original registers and the BTs exist, a wise researcher will consult both.