Your Basal or "at-rest" temperature can be observed to assist you in gauging your fertile status.
The basal temperature will tell you when ovulation has occurred after it has occurred . It is useful for telling you that you have ovulated but not much good at warning you that ovulation is going to happen.
For general observation and charting purposes, it is not necessary to take your basal temperature. But you may wish to if you want to know more precisely when you have ovulated. This will help you in determining your mucus pattern.
The Basal Temperature method works on the fact that your "at rest" body temperature will drop slightly before ovulation and then rise by 0.5 degree Celsius / 1 degree Fahrenheit after ovulation has occurred. It remains raised until you bleed, when it will drop again, to start the cycle anew.
Your Basal Temperature is measured by taking your temperature in the normal fashion each morning as soon as you wake up and before you do anything else (including going to the toilet, having a cup of tea etc.). Write the temperature down on your chart, and soon you will notice a pattern.
As it can be tricky to spot the rise, see a Natural Fertility Counsellor or consult a specialist book e.g. Natural Fertility if you want to use this method comprehensively. Hormonal Forecaster also has information and links to help you track temperature.
And remember many factors can affect temperature for example alcohol, illness, travelling across timezones so it is not always a reliable indicator. Ideally use temperature observation to back up your other fertility observation.