The symptothermal method is a method of natural family planning (NFP). It combines the observation of body temperature and cervical mucus to evaluate the cycle. 

A brief explanation on cervical mucus / temperature and cycle:

The maturing ovum produces the hormone oestrogen. Oestrogen changes the consistency of cervical mucus, the more oestrogen, the more like egg whites the cervical mucus becomes. After ovulation has taken place,, the corpus luteum (the shell of the released ovum) is degraded, which produces progesterone. Progesterone changes the body temperature; it rises the more progesterone is produced.

You can learn more about the symptothermal method here.