Without getting "overly technical," Wikipedia describes BTUs (British Thermal Units), as the traditional measurement of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
The BTU output of your gas fire pit table will determine how effectively it will heat the space around it. The higher the BTU rating, the more heat it will produce.
Consequently, your needs will determine how many BTUs you need based on your time outdoors in colder weather.
A lower BTU-rated fire pit table (35-40,000 BTUs) will be perfectly adequate if you live in a warm climate where evenings rarely get cold.