CO+2H₂--^{340 ATM}--------->CH₃OH

WHAT'S THE MEANING OF 340 ATM

Atm stands for atmospheres, which is a unit for measuring pressuring. In a chemical reaction, it is generally the reaction conditions (like required temperature, pressure, presence of a catalyst etc) which are mentioned above the arrow. Thus, in the given reaction, 340 Atm means that the said amount of pressure is required for converting the reactants into the products. 

@ Dimple, very nice answer. Keep it up!