It's called Body Basal Temperature (basically, it means taking your resting body temperature). You can buy a special thermometer at Boots, or local chemist (not too expensive about £5 - £10).
What you do, is every morning, when you wake, you take your temperature (orally - apparantely you can do it vaginally
as well!). You write down your temperature, some kits come with a little book (so you can join the dots to see how your temperature rises and falls) or you could join a fertility site on the internet like Fertility Friend and it charts it for you (you just stick in the numbers).
Temperatures should look something like this:
After AF - the temperature goes down.
After Ovulation - just after you ovulate, your temperature rises suddenly and obviously. This is due to an increase in progesterone levels in your body. It stays high until the time round about AF being due.
AF - your temperature dips again, gradually going down to pre-ovulation levels.
Pregnant! - If you are pregnant, your temperature will remain high and may even start to increase round about the time your AF is due. They say that if your temperature remains high for more than 14 days post ovulation, then there is a good chance you are preggers!
Hope this helps!