An LNG train is a liquefied natural gas plant's liquefaction and purification facility.
In order to make it practical and commercially viable to transport natural gas from one country to another, its volume has to be greatly reduced. To obtain maximum volume reduction, the gas has to be liquefied (condensed) by refrigeration to less than -161 °C (the boiling point of methane at atmospheric pressure). This process also requires very strict safety measures and precautions during all liquefaction stages, due to the flammable nature of the gas involved.
Since the numerous impurities that are naturally found in the raw gas freeze at low temperatures, and would thus block the cryogenic section of the plant, the gas has to be purified before it can be cooled down to cryogenic temperatures.