A basal thermometer is a digital thermometer showing two decimals, (for example 36.29). It is more sensitive than a regular thermometer, as it measures the to two decimals. This is very important when measuring your basal body temperature, which rises by 0.2-0.45 °C after ovulation.
This is why you need to use a basal thermometer with Natural Cycles, and not use a regular thermometer. A fever thermometer measures only to 1 decimal.
It will take a few weeks of measuring for Natural Cycles to understand and get to know your unique cycle. If you just got started - keep measuring and adding data so Natural Cycles can find ovulation.
When you sign up for a yearly subscription, you get a basal thermometer included in the price.