The basal metabolic rate - or about the number of calories your body would consume if you were living the non moving vegetable life. 

For men and women in imperial "freedom" units this works out to:
$$
\begin{align}
\text{Male BMR}&=66 + (6.2 * \text{Pounds}) + (12.7 * \text{Inches}) - (6.76 * \text{YearsOld})\\
\text{Female BMR}&=655.1 + (4.35 * \text{Pounds}) + (4.7 * \text{Inches}) - (4.7 * \text{YearsOld})
\end{align}
$$
To get a better idea of what your actual daily caloric burn rate is so you can adjust your nutrition to maintain a healthy weight your BMR can be expanded upon by using your Total Daily Energy Expenditure multiplier or [[TDEE]]. The TDEE factors in the activity levels in your lifestyle to provide a better idea of where your caloric needs are.