BBT is your Basal Body Temperature. It is the lowest temperature attained by the body, usually during sleep. It is generally measured immediately after awakening. Use a thermometer with an accuracy of 0.1° or better.
Ovulation causes an increase of 0.5 - 1°F (0.25 - 0.5°C) in basal body temperature. This small increase in BBT occurs within 48 hours of ovulation and remains elevated until your next period. Monitoring of BBT is a way to determine íf you are ovulating and if so, on which day of your cycle this occurred.
Records of basal body temperatures can be used to accurately determine if you are ovulating, and if the length of the post-ovulatory (luteal) phase of your menstrual cycle is sufficient to sustain a pregnancy. Talk to your doctor if your luteal phase is shorter than 12 days, or if you don't seem to ovulate.
In normal cycles you should see an increase of the BBT after ovulation. When the next menstruation occurs, the BBT drops again. If you are pregnant however, the BBT will stay high and you won't menstruate. If the temperature stays high, even after you would expect your menstruation to occur, you should try a pregnancy test.