What are the metal displacement reactions? How can the direction of such a reaction be predicted?

Those reactions in which a more reactive element displaces a less reactive element from the salt solution of the less reactive element are called displacement reactions. 

For example, zinc is more reactive metal than copper. Hence, when zinc is added to copper sulphate solution, it will displace copper from copper sulphate, forming zinc sulphate and copper metal instead. Following is the chemical equation for the reaction :

  
These reactions are uni-direction reactions