BBT stands for Basal Body Temperature. BBT is a healthy body's temperature when at rest.
Slight variations of BBT can be used to determine when ovulation has occurred. BBT is commonly tracked by people who are practicing a FAM (Fertility Awareness Method) of family planning. Measuring BBT must be done under very controlled settings, and is typically measured during sleep, or immediately after waking up, before sitting up in bed.
To find out more, see the informational text in Clue about BBT.