MMBTU, or MBTU, stands for one million British Thermal Units (BTU). A BTU is a measure of the energy content in fuel, and is is used in the power, steam generation, heating and air conditioning industries. One BTU is equivalent to 1.06 Joules.
Natural gas is usually measured in BTUs.
"MM" stands for one million - it originates with the Roman numeral M, which stands for 1,000. MM is therefore "one thousand thousand," or one million.