Your BBT, your body temperature at rest, is a critical fertility sign because it is the only sign that will tell you definitively that you ovulated. It also is the only sign that will let you pinpoint (to as close a degree as possible) when ovulation occurred. All your other signs tell you only that ovulation is approaching.
Charting your BBT will also tell you how long your luteal phase is so you will know when to test for pregnancy, when you may be pregnant and also if it is sufficiently long to allow for a pregnancy to occur. It is best to use your cervical fluid and other signs in conjunction with charting your BBT to get the most out of charting and to get the most out of Fertility Friend's analysis tools.
After ovulation, the corpus luteum (the remains of the follicle that released an egg at ovulation) produces progesterone. Progesterone causes an increase in your body temperature that is observable when you measure your BBT with a special BBT thermometer just upon waking in the morning. This increased progesterone after ovulation is what lets you see a thermal shift on your chart after ovulation.